I wonder if they wait for the right moment (e.g. when Android gets replaced with whatever Google tries to replace it with) for a new play on mobile or if they really are "done" with it.
I think there is a still a possibility that Microsoft expects the phone and PC markets to merge and hopes to be there with welcoming arms when it does, but I'm getting the impression they won't build new hardware to do it if they can't convince other manufacturers to join in.