I haven't seen all the nominees, but the ones I did see -- Train Dreams and Sinners -- were, to our eyes, profoundly better films than OBAA. I'm in particular interested in seeing Hamnet soon; everything I read about it puts it in the same category as TD and S.
OBAA was the safe Academy pick, and so that's what they picked.
It's one of those movies where almost everyone looks like they just really love being on stage ("isn't cinema lovely?") and where the writers have an idea of what cliches they're trying to work with but can't land them into an actual story, even a story made out of cliches.
They did throw some serious money at this film, though, so I can see where people would have strange expectations.