1. The game catalog is totally the most important thing and they don't have any really big games. I'm not a Fortnite person, but they don't have that, nor Apex, nor PUBG. Battle Royale may not be the biggest thing in gaming anymore, but this is the type of game that at least a lot of people enjoy and they have none on the platform.
2. They sold Stadia as being able to achieve higher quality than local hardware at a fraction of the cost. Yet it's been shown multiple times that the games running on Stadia are running at less than promised quality, with techniques like upscaling being used for 4K instead of native 4K: https://arstechnica.com/gaming/2019/11/google-addresses-comp....
3. As a person who wants to play with a keyboard and a mouse on a computer, I get really frustrated that the Stadia output resolution is fixed. You can't output 1440p to a monitor at all, and 4K is only available if you have a Chromecast. Why?
I've also tried Shadow and GeForce now. Shadow is easily the best in terms of quality, because you can run at arbitrary resolutions and pump the bandwidth up to a max of 70Mbps down, which is astoundingly good looking compared to what Stadia offers. GeForce Now is great too, but point 3 applies to it: you can't run at non-preset resolutions, which really stinks if you have a bigger monitor with a higher native resolution. However, both Shadow and GeForce Now leverage existing game libraries, look better, and are generally more approachable than Stadia's "rebuy everything, run at lower quality, and lack significant features" plan.