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.