That would be declaring a full on browser war with Google. I expect MS don't want to do that yet. It remains an option in the future I suppose.
As I understand it Chromium is a lot easier to wrap your own GUI around than Gecko. XUL is still not completely decoupled, separating them completely is an ongoing effort I think.
Maybe when the embedded Gecko/Servo engine is production ready would be the time for MS to reconsider.
Google, Microsoft new "bestie" in this collaboration, will screw them over once it's clear that Microsoft "can't turn back" from using Chromium or even fork it.