1. First-party AAA games are a huge investment (in time and money), especially for a company without any experience in building them. Starting a first-party game studio is a huge risk (financial and planning) that I'm not surprised didn't pan out.
2. Stadia has a reputation as the "indie game streamer" platform in a lot of circles after such a large % of their first games were older and from smaller dev cos. The unpolished first-party AAA game (if it ever came out) wouldn't do much to help this reputation; focusing on partnerships and getting new releases on the platform seems like it'd do far more for the service's future.
3. Unpopular opinion but I don't think exclusives should exist for any platform. It may be good for business but it's bad for most gamers.
4. Politically, it feels like we're about to move into a period where companies need to be way more careful with monopolistic moves into new markets. "Owning the platform and the content" seems to be a wedge-prone position that a lot of companies are worried about (Amazon, Apple, Google, etc) and this kind of just clears Stadia/Google from having to worry about that on "their console".
FWIW I have almost 100 hours on Stadia and 49 friends online in Stadia at the time of writing this comment, but YMMV, I am most definitely in some kind of Stadia-bubble, I probably have some confirmation bias, and this may also be wishful thinking based on the fact that I want Stadia (and cloud gaming as a whole) to succeed.