A) Major devs get it early
B) Google has cut so many APIs out of it and pushed them into Play Services (Google only, closed)
C) So much of the stock Android experience has been completely ignored and neglected now that Google launched the Pixel line. Unlike Nexus, Pixels are filled with "exclusives", and instead of just being the best place to use android, it keeps features from others.
As a pixel owner, none of the exclusives seem part of android. It’s been stuff like camera effects, which I associate more with apps. As someone who loved nexus and wished pixels were more like that, I’m disappointed too with the proliferation of play services apis. However, I don’t mind that pixels are competitive, as I think it fosters innovation. If pixels were like nexus, and just a pure android experience, there would be nothing new in the last few generations. Phone OS innovation has plateaued.
See, the issue is over time they have carved more and more out of stock AOSP and made it closed.
The AOSP camera is garbage, but it doesn't have to be. I think the Pixel line could still have done great with Google backporting these Improvments even on a delayed release window. But you have people literally stealing the camera apk and manually patching it to run on other devices because Google won't do this.
Not to mention the sorry state of stock apps like calculator and calendar, notes, all ignored in favor of a competing google service... Which Google sometimes still ignores (Keep)
I'm happy Google made Pixels, but I think in the switch from Nexus to Pixel, the loser was the AOSP program, which now feels more like a skeleton that requires a full OSS team or corporation to finish it. Too many basic QoL changes are never backported.