Because OAuth 2.0 sucks and got all bloated. The 1.0 version worked great for everybody but super big companies like google and facebook. But then big companies came in and made it a bloated standard that doesn't meet the needs of most users.
That's not entirely true. OAuth 1.0 and 2.0 didn't cover much, and everything was implementation specific. Both standards only cover authorization, while the majority of consumers wanted to use it as authentication.
Big companies had to roll out their own authentication layer on top. When OIDC came into play, it was already too late.
