Microsoft is the OS vendor. They aren't the only stakeholder in this space. There was, for a time, hope that the UEFI Forum, or an entity attached to it, or perhaps Intel itself might step up and offer to be a central key authority for the UEFI secure boot standard. They didn't. Microsoft did, and even offered to sign keys for other OSes.
I get the impression that it's a lot easier to be the first at bat when you own the ballpark.