1. Too late, too slow
Microsoft and Symbian (along with Palm and Blackberry) were smartphone pioneers, of course. I still remember handling the first 'XDA' running Windows Mobile, and recognising that this was the future. That was in 2002, with 'large' touchscreen, a full five years before the iPhone was even announced.
So there was momentum on Microsoft's side, but Windows Mobile was very much a sideline and development was glacial. HTC and others did what they could to slap skins on top of it, but results were always a kludge. And by the time Microsoft felt it had to really compete, after three years of iPhone and two years of Android, Windows Mobile was so long in the tooth that it had to be completely gutted and rebuilt, for Windows Phone 7. The UI was superb, the underlying OS less so and it wasn't until Windows Phone 8.1, four years later in 2014, that 'Windows Phone' became a viable software product that manufacturers could really push and that reviewers could recommend.
2014. So it took seven years to really respond to Android and iOS in a mainstream way, seven years in which these other two mobile OS had been building up their maturity, scope, applications ecosystems, and in Android's case, signing up a hundred different licensees. While Microsoft had Nokia and... not that much else. Bringing us to...
2. Nokia was too dominant for too long
Although Nokia wasn't in the initial batch of Windows Phone licensees, when it did 'sign' with Microsoft (via ex-MS man Stephen Elop) it put its entire smartphone future into Windows, whereas HTC, LG and Dell (from the initial batch) were just playing with the OS by comparison. As a result, well over 90% of all Windows Phone sales were of Nokia handsets before too long and this only served to push other licensees away. The brand 'Lumia' became synonymous with the OS and even with the might of Nokia's branding there just wasn't enough momentum.
Yes, Apple does very well with its single-brand OS, but that has a 'fashion' association that's unique in recent tech history. Windows Phone required more companies getting involved in a serious way.
3. Nokia's focus on the low end
If Nokia had aimed for the high end, putting most of its efforts into its flagships, then I think Windows Phone would have been taken more seriously. As it was, we had the Lumia 1520 (terrific handset, ahead of its time, but unheard of in the High Street), the Lumia 1020 (widely marketed as a camera-centric device, and rightly so, but every spec other than the camera was decidedly 'old' and it's not surprising that it was one compromise too far for the mainstream), and the Lumia 930 (utterly flawed in terms of display - no always on facility, and chipset - Nokia really screwed up the thermal management).
But the mainstream smartphone world viewed Windows Phone and Lumias as low end, cheap devices, fuelled by a never ending sequence of launches of horribly small and underpowered Nokia handsets. The 4xx, 5xx and even 6xx series all struggled to run fluidly as the OS and applications grew and as people's expectations of how fast a smartphone should be increased over the years.
I realise that this might be a personal thing (i.e. for me), but if there had been a Lumia 1030, with larger display, a Snapdragon 8xx chipset, sub-second oversampling times, in 2014 to accompany the Lumia 930, then I think Nokia would have had some serious traction in the phone enthusiast community, with interest that might have trickled down more to the low end (the 'friends and family' effect).
4. Not in the USA
Yes, Microsoft is a US company, but Nokia is Finnish and the Nokia brand has never dominated in the USA in the way it has in the rest of the world. When Symbian OS had 60% world market share in 2008 or so, it was almost unknown in the USA. And when Nokia's Windows Phones were starting to pick up traction in Europe (20% or so at one stage in Finland and Italy), the brand and OS combination were a non-starter in America.
From 2000-2010, this was because Nokia largely refused to compromise its handsets and OS for the US carriers, which were very arrogant in pre-iPhone times, in terms of their demands; in the Windows Phone age, it was because everything got drowned out by the ever-pricier iPhone and the way the US phone contract model dominated, making Apple's handset barely more expensive to own and run than a plastic budget offering.
Why is the USA so important? Because it just is - to the chagrin of a UK writer like myself. The big money, the largest traffic flow, the big advertising budgets, are nearly all in USA-based online publications (The Verge, Engadget, Wired, Mashable, the list is endless). Making it somewhat unfortunate that the USA smartphone scene is so skewed towards what is a minority OS worldwide (Android has 80% market share now, but you wouldn't know it when walking around a US city - you'll mainly see iPhones).
5. Ignored by Google, Snapchat, etc.
One of the key gaps in Windows Phone's (and then Windows 10 Mobile's) ecosystem has been a lack of official Google applications. The problem is that the Chrome browser, YouTube and Google Maps, to name just the biggest three, are ubiquitous in 2018 in most homes. Yes, they're (respectively) a browser and information available via a browser, but the latter two in particular are much better as standalone clients. But because Google is behind Android, Windows Phone's (and Windows 10 Mobile's) biggest competitor (iOS stands apart in many ways), why should Google put any effort into coding up first party clients for its services for a rival OS, especially one that's very much behind it in real world use? Why help a challenger challenge more?
Why indeed. AAWP readers will know how good Edge often is, of course, and that there are excellent third party YouTube and Google Maps clients, but it's not quite the same for the man in the street, who expects first party clients in the on-device Store.
Google's motivation for ignoring Windows Phone may have been professional rivalry, but other popular services have ignored Microsoft's operating systems for different reasons. Snapchat is a good (but not the only) example - CEO Evan Spiegel has apparently often spoken about his hatred for Microsoft and Windows and, while I think 'hatred' sounds too strong to be an accurate quote, there's certainly some dismissal going on. So no first party Snapchat client and lawyers sent to the developers of any third party Snapchat clients (e.g. 6snap) for Windows phones.
6. Under-using the Metro UI
Around nine years ago, Microsoft came up with the Metro UI ideas for Windows Phone 7 - large, bold titles, side-swiping pane UI, bottom-of-screen controls, grouping information between applications into 'hubs' (especially the People hub), deep linking from one app to a specific position in another. The look and feel - and functionality - was different to Android and iOS and there was a genuine feeling that Windows Phone could take off, carving out a sizeable niche as the 'third partform'.
What actually happened was that development resources for Windows Phone 8.x were limited and, stung by disappointing sales market share, Microsoft ended up folding mobile into its new, all-encompassing Windows 10 ('Windows as a service') project. Which is pretty cool in its own right, with applications that run on the phone just as they do on tablets and laptops and with twice-yearly OS branch updates.... yet it's not as immediately ground breaking as the Metro UI on the earlier Windows Phone 7.x devices. Windows 10 Mobile is all about function and less about 'wow'.
Which is a shame - live tiles aside (some of which have stopped working now), all people see of W10M in 2018 is a phone interface that looks a little staid and which runs at half (or less) the speed of other phone UIs.
7. Under-developing Windows 10 Mobile's USPs
Leaving aside the 'staid' interface, W10M did have USPs, not least the aforementioned UI and UWP compatibility with many other form factors of Windows 10 computer, plus the Continuum system to utilise external TVs and monitors as larger native displays. But in the real world, no one really cared about the former, other than computer science geeks like me. And the latter (Continuum) was never developed to its fullest potential - heck it was hardly tweaked after the initial release.
Compare Continuum with Samsung's 2017 DeX system, which isn't as elegant (it's an add-on windowing system rather than being a literal extension of the main phone display) but it works much better, thanks to far faster chips powering the experience and to more development, including windows that can overlap, be minimised, and so on.
The fact that Microsoft got 'there' first two years before is, ironically, forgotten by most commentators now. But development of the Continuum system in Windows 10 Mobile got axed at the same time as development of the rest of the OS, which is a crying shame.
8. Sheer bad luck in terms of timing
Yes, Microsoft was late to the new wave of smartphones with its Windows Phone OS, but it has also been unlucky in terms of timing in the context of the industry.
For example, PWAs (Progressive Web Applications) are the latest thing and could in theory have been the saviour of Windows 10 Mobile, but development of the integral Edge browser was halted (with the 'feature2' branch) under Windows 10 Mobile with only partial PWA support. Which is why we get PWAs highlighted here on AAWP but with caveats like "of course, there's no support for notifications, or background operation". If Edge development for W10M had been carried on for just one more OS branch then PWAs would be dramatically more powerful.
Then there's the Internet of Things, cloud-connected home accessories that have really started taking off in the last couple of years, with the smartphone being the obvious personal computing device with which to control them. Yet not Windows 10 Mobile, since the OS had effectively been mothballed in terms of development just as the IoT scene was exploding. The 'app gap' for IoT under W10M is not just a 'gap', it's a yawning chasm.
9. No investment, no payments
A huge part of the modern smartphone experience is being able to pay for things with just your phone. Even if you don't do this every day, it's cool to think that it's possible and I'll bet you'll have experimented. If you have an iOS or Android smartphone that is. It turns out that implementing 'tap to pay' from a phone is a lot of work. Not just on the application itself - that's quite easy, but in working with multiple banks in multiple countries to hook your APIs together with the rest of the financial world. And with proven, reliable (and quick) operation.
It's a lot of work and Microsoft simply didn't put enough effort into this area. Microsoft Wallet/Pay was rolled out to some degree in the USA as a trial, but without a succeeding mobile platform of its own the necessary investment by Microsoft to go further afield didn't really make sense. iPhones use Apple Pay, Android phones use Google Pay, and so on. Yes, Samsung has its own version too, but then Samsung has enormous reach and resources.
10. The big shutdown
Microsoft's decision to buy Nokia's Devices division could have worked if it hadn't then shortly started shutting down everything it had just bought, culminating it stopping making new phones at all mid 2016. In the main, the Ballmer/Nadella change at the top was to blame, with very different visions of what Microsoft should be doing. And while I sympathise with the idea that the 'old' Nokia factory/support/sales infrastructure had to go, that doesn't mean that first party hardware had to go - there are many different ways Lumia production could have continued. Look at the Surface hardware, for example.
Plus designing a phone, getting it made and distributed is far easier these days than it was a decade ago. Look at the multitude of willing Chinese OEMs, heck look at Wileyfox, which 'made' a brand new Windows 10 Mobile smartphone in 2018 and which is still being updated in line with the top Lumias.
Ten reasons then - and I dare say that you can add more in the comments below. But ten is enough - take them cumulatively (over eight years, admittedly) and it's easy to see why Windows Phone and then Windows 10 Mobile ended up being a technological backwater rather than a mainstream contender.
Could things have been different? I think so. More belief by Microsoft in its own mobile OS, better management at Nokia, more Windows Phone licensees, all resulting in more engagement with the developer world, more applications, and so on.
I'd normally finish with an aphorism like 'hindsight is a wonderful thing', but in this case we at AAWP (and many other writers and broadcasters) called most of the mistakes above as they were happening, yet Nokia's and then Microsoft's management seemed set on a dithering sequence of bad decisions, culminating in the virtual disappearance of Windows on phones.