Part of the problem is that they never delivered on some of the features that really would have distinguished it from being just another streaming service. Features like youtube integration where a video of someone playing the game might autopopulate a stadia link to start playing that same game right now.
But the main thing they got wrong was the library of games. The success of just about every console launch has lived or died by the # and quality of games available at launch. Only in this case they were launching with games that were already available for anyone who wanted them. There was no additional market to tap into, existing PC games already owned the games Stadia was selling.
This meant the value proposition was either:
1)the low cost of entry for non-pc gamers. This is a relatively hard sell though. There aren't a lot of mass market games that you can't also find on cheaper consoles, so converting console gamers is rough, leaving you with non-gamers to bring into the fold, which is even tougher.
2)Convenience for existing games that already owned the same games. This is where Google really missed the ball. They should have used their giant size & resources to forge agreements with the big publishers that would let folks who already owned games on Steam to play them on Stadia. Pay them off the same way console makers pay for exclusive titles: a cost of doing business. Or a cut of ad revenue for ads shown during live streams of their games. Or even negotiate for consumers to pay a small 1-time fee to add a steam game to their account: This is a bit how Apple was able to be so successful with iTunes early on. Lot's of people already owned physical copies of music they ended up re-purchasing on iTunes because at $1/song it was pretty cheap to buy a "best of" sampling of your existing collection for the convenience iTunes offered. I don't think I would have purchased a Steam Deck if instead I could have spent a few dollars activating games I already owned on Stadia instead. Heck models for this sort of thing already exist: I've paid $1.99 to "read" an audiobook version of a book I already purchased from Amazon a bunch of times. There were multiple ways Google might have overcome resistance from existing gamers to adopt Stadia due to not wanting to repurchase their favorite games.
Instead the "free" games included in a sub were either a) already owned by the target audience or b) little better than shovelware. You got very little for subscribing