I think it comes down to their vision changing (flip flopping really). For awhile they moved away from it due to Unity, but lately they have started fully supporting Monogame (as in you can use it to make games for all their products).
But after they moved away from XNA, Monogame started doing the work. Which means there is no room for Microsoft to Reenter the project (which I think is good overall).
A good question to ask yourself would be:
What would Microsoft be able to add to the Monogame currently? If the answer is "not much" then that is why Microsoft has stayed out...
If there ever comes a day where XNA/Monogame is part of Microsoft's vision, and Monogame is no longer being developed by the community then I am sure they will jump back in