Huawei depended on the proprietary bits of Android ("Google Play Services"?). In contrast, Microsoft is knowledgeable enough to use only the permissively-licensed parts of Chrome as the point of departure for their fork.

Also, if Huawei were to fork Android they'd be responsible keeping their fork of a huge code base secure by push security updates, and Microsoft is much more likely to be able to do that competently than Huawei is.