I know folks at AAA game studios/publishers and the primary reason I've repeatedly heard is: Stadia requires that we port to Linux.
Completely insane requirement. Steam, meanwhile, is making Linux gaming better than ever with their investment in Proton (which is incredible, wine has never been so good) and Nvidia's game streaming service "just works" and you bring your own games (just log into steam).
Stadia is screwed from the get-go by thrusting inane requirements onto their potential partners. If they had huge market power, maybe it'd fly, but it's effectively a new console as far as publishers are concerned.
Instead it should just work with PC games. End of story.