Maybe he was overcome with guilt and shame...

the only division at MS that didn't get stuck in 2000 has lost its head ... may we get the same from the office team so whoever decided that supporting live tiles is a no go even in office 2013 can go out and wash cars instead of doing abysmal decisions of global scale in the future?

Ooh, maybe they will bring back XNA?? Heh, I'll keep dreaming . . .

I think the term I've heard Sinofsky described as most often is uncompromising. And say what you want about him as a person, his accomplishments with office and win7 were stellar. Win8 compared to Win7 is a mess in terms of providing a slick unified dev environment and user experience on the desktop, and not something I imagine a perfectionist like Sinofsky is overly happy about.

My guess, and it is only a guess, is that Sinofsky did what he was told wrt to win8 development in stead of his usually M.O., which is to follow his own path. He did this with the promise of being named Ballmer's successor. As win8 entered the final phases, that promise was withdrawn for whatever reason, and so Sinofsky stuck it out so he could see the Frankenstien win8 to completion because he is a professional.

Once that was done, there was no reason to stay as Win8 wasn't his vision, and the CEO job wasn't going to happen.