The video and text were from an AMA on reddit, according to WMPU:
Hello! These are all super common questions. But let me unpack some of your statements and dissect them a bit.
- Why not let the user swipe?
- Hamburger – bottom is better if you interact a lot
1) Swiping sucks. It hides content. Let’s say you’re in Format and you want to get to something 5 tabs away. Five swipes is an unacceptable series of interactions. The carousel model has been disproven repeatedly, every single decade, for several decades. We have the data. It’s a dumb interaction model, full stop.
2) It turns out bottom is not better. You’d think that something 3 pixels from your palm would be easier to reach than something in the middle of the phone. But nope. The way average people hold phones means the middle of the device is the best location. Both bottom and top require your hand to make a bit of a shift to reach. (this is why swiping on items to get options like “flag message” or “delete” is popular. Make the gesture contextual to the item itself and you don’t make people reach to use it. And this is, again, why reserving swipe at the app level for navigation is not a good model.)
2b) You don’t use the hamburger very often. Most of your time is spent reading, and then some of your time is spent manipulating the content. So if we put the hamburger on the bottom (which no major app in the whole industry does) then the app bar on the bottom would have to get larger in Office. And in the case of Outlook, you’d have to actually put a bar down there in the first place.
This is just a small extract, so do click through to read the originals, ending up at the full AMA. There's also the issue of alignment with the rest of the industry, explained in the video below:
Perhaps the biggest 'aha' moment in the linked explanation was that the 'Metro'/panorama interface made perfect sense when screens were small enough that thumb side swipes were trivial - but they're a lot more work on 5"-6" displays, so it seems that goalposts do sometimes move...(!)
If you'd like a taste of how Windows 10 for phones will look and have a non-essential smartphone then do grab the Insiders build.