I suspect that when they sat down and realistically summarized their strengths and weaknesses they figured out the game has probably been lost for them on mobile and tablets. The next most logical step is to protect your enterprise turf. I would not be surprised if they spun out Xbox and perhaps even Windows Phone.
Microsoft has probably lost in mobile. Their next best play here is to be on every platform with their ecosystem apps and guard against Google or Apple getting a foothold. They'd be foolish to continue to push the Windows Phone boulder uphill.