A better explanation is that people didn't love those movies as much as Reddit did.

I mean these films were well received by people outside of Reddit. The actual explanation is something you started to explained but veered off of for some reason. Studio's only get behind one or two films, and will rarely push two movies in the same category in case it splits the difference.

Paramount chose A Quiet Place over Annihilation
A24 chose Eighth Grade & First Reformed over Heredtirary
Annapurna chose If Beale Street Could Talk & Vice over Sorry To Bother You and,
Summit chose not to push Blindspotting because it didn't stand a chance.

The awards have little to do with general opinion and more to do with how hard they're marketed to Oscar voters who have admitted in the past, they don't always even seen all the nominated films and just vote for one's they have seen and know.