There were plans afoot for Carbon to be rewritten from the ground up, and to implement other standard Windows Phone features. To do this, the team was looking for local Windows Phone developers. However, hiring such staff is said to have been too costly given the revenue from sales of Carbon. Rather than have a version of the application that would no longer be supported, the team decided to withdraw the application outright.
Since we stopped updating the app long ago, it is the best way to stop sales, there are no options for us to keep the app in the Marketplace and only stop new Purchases, so to further stop dissatisfaction of new users who won’t get support or updates we pulled it out of the Marketplace.
It would be easy to point to this as evidence that the Windows Phone user base is not large enough to support professional developers. However, this is just one case; we would need more instances of similar cases to start drawing any conclusions. Furthermore, it's evident that the Carbon team has been particularly unfortunate, and has had to make a business decision weighed against its other projects.
If you're a Carbon user, please leave your thoughts about this application in the comments.