As was the case with the old Soviet Union, Microsoft decisions frequently only make sense when viewed through the lens of internal politics -- of one apparatchik trying to stick a knife in the back of another, in order to increase their own power.

My guess on this one: the new Metro-style app platform was the work of Sinofsky's Windows Division, while traditional XAML app tooling belonged to some other division. In such a scenario, if XAML could be deprecated and Metro-style apps made the One Way to develop apps on Windows, it would mean that a big slice of Microsoft's business that had previously belonged to some other exec would now belong to Sinofsky.

(A turf war, in other words.)

If the idea of a Microsoft executive being willing to pursue a course that would hurt Microsoft just to increase his own power seems unrealistic, I would refer you to the Iron Law of Institutions (http://www.tinyrevolution.com/mt/archives/001705.html):

The people who control institutions care first and foremost about their power within the institution rather than the power of the institution itself. Thus, they would rather the institution "fail" while they remain in power within the institution than for the institution to "succeed" if that requires them to lose power within the institution.

Sinofsky had a reputation for not working well with anyone outside of his team[1]..Net (including WPF) occurs outside of the Windows team.

[1]: http://allthingsd.com/20121113/in-the-sinofsky-departure-bil...

And indeed it wasn't popular at all with devs: http://www.zdnet.com/article/windows-8-developers-are-shunni...