I kinda doubt there's going to be any outside contributions to this project, so any non-exclusive license would allow MS to rerelease this under a proprietary license - even the GPL. They own the copyright, so they can release it under as many licenses as they wish.

With software there's way lower barrier to entry and way higher chance of having the patches accepted. With art... Good luck convincing MS that your version of the emoji is worth the hassle of losing the ability to just change the license at any moment.