Reddit has a large enough userbase that duplicate communities can each reach a sustainable size without interfering. The fediverse userbase isn't large enough to sustain even a single community for some topics, let alone duplicates. I'm in plenty of communities where there are lots of low value posts that would normally be consolidated into a single stickied post for the community but there isn't a large enough userbase to make a stickied post worthwhile despite there being multiple communities for that topic.

Also, reddit is a centralized system. A decentralized system is going to have problems that a centralized one doesn't