It is just more beneficial for MS to use webkit. They neuter Googles ability to use its dominance to give preference to its own services and interests. Having a minority browser engine (which Mozilla would still be) gives no strategic advantages at all. IE was only strategically useful when it was dominant. Now it is just a resource sink.

MS supports linux now. They don't care about past rivalries, they just do what makes business sense to them.

The lack of diversity might be bad for the web at large though.