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.
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.