I think it's just that "Windows X for Y" where Y is a possibly trademarked entity works better for Microsoft in general. Maybe it wasn't risky for Linux, but if

I think it's just that "Windows X for Y" where Y is a possibly trademarked entity works better for Microsoft in general. Maybe it wasn't risky for Linux, but if they ever want an other "Windows subsystem for Z", where Z is a trademarked name, then this puts them in the clear.