Valve has the resources to hack Proton to make things work, others just want an OS they know will already run Windows games without much fuss. Valve specifically wants to move away from Windows because of fears of anticompetitive behavior from Microsoft. They're not just doing it from the goodness of their hearts. Microsoft would like nothing more than the Steam store crushed and all its games moved to their own walled garden.
A big reason to move away from Microsoft is also lack of licensing fees, which the other companies can definitely get behind. They'd have to make their own store and front end most likely, but proton is basically all done for them and is already in a shippable state that "just works" for users.