IMHO: it's driven by multiple MS Windows Teams, the following blog explains it very well:

http://arstechnica.com/microsoft/news/2011/06/windows-8-for-software-developers-the-longhorn-dream-reborn.ars

Quote:
 "... WinDiv, dissatisfied with DevDiv,..."

My personal conclusion: WinDiv created something like an 'Anti-CLR', named Windows Runtime (WinRT), but IMHO it's an architectural disaster! For example:
They extracted a confusing subset of the Win32 and COM API, see MSDN::
http%3A%2F%2Fmsdn.microsoft.com%2Fen-us%2Flibrary%2Fwindows%2Fapps%2Fbr205757.aspx 

and now they call this 'reinventing windows'!