MG Siegler speculates on the reasons for Sinofsky's departure from Microsoft:
Sinofsky was the driving force behind the “no compromise” approach to Windows 8. I believe that approach is at the heart of the ultimate problem with the OS. As two separate halves, Windows 8 and Metro seem fine. As a whole, the OS seems like a schizophrenic mess. Microsoft should have copied the Apple approach with OS X/iOS, keeping them separate and slowly merging them over time by taking the best of both.
To be fair to Sinofsky, Ballmer was also a big advocate of the "no compromise" approach.