I don't think it was your other reasons. You could use other controllers on stadia- they even made it a selling point that the only thing you had to buy was the

I don't think it was your other reasons. You could use other controllers on stadia- they even made it a selling point that the only thing you had to buy was the game. And when connected via USB stadia controllers worked fine with Steam games (at least on Linux). Another possible reason is that it wasn't seen as a high enough priority compared to other things they wanted to do.