Is it expensive? Is Stadia not an established brand like Xbox or PS5? Are they afraid of cheating like those on pc and Nintendo switch using a lag switch? Wouldn't google stop it if that was the case? I'm just confused as to why Stadia players are not being taken seriously like PS and Xbox players?
I think you need to ask the developers this ^^
From my perspective it might be, because Stadia is new and they don't have the resources atm to spend on developing crossplay for a new platform. But could also be that the CEOs/directors of the game have not set that as priority in comparison to bringing new features or fixing bugs for the game.
Best would be to let the developers know about it, so that they can work on it. The more people do that, the more it could become priority.
Just a guess, but best we could do atm.