It didn't reach mainstream adoption because most people who care about games didn't think it was a good deal and input lag was a big part of the reason why. And people who don't care about games... didn't care about Stadia for obvious reasons. The target demo for Stadia was basically people who have a mild interest in gaming but not enough of an interest to already own hardware suitable for running games themselves. That was never going to be a profitable niche.
Why buy games full price? All my gamer friends say its crap!
Google shuts things down, why invest?
The tech was pretty good for casual gamers, it also pushed linux gaming in the AAA studios.
I think a lot of negative sentiment also came from gamers thinking that Stadia intended to replace their gaming setups.