Android as Plague Carrier
My take is that they couldn't overcome some of the basic security flaws of Android.
Microsoft saw how the desktop became totally vulnerable security wise, mostly becasue they felt they had to support legacy systems. I think they realize that if they once again become a plague carrier it will lead to them becoming even more marginalized.
Adding Android emulation to their mobile platform is just another legacy emulaton for them, and one they have to be very very careful about how it is done.
It may encourage some developers to port, but any good developer will have tear it down and develop again to run Windows well.