I think it's just discoverability of content, and probably some UX. Mastodon isn't really a great show of what ActivityPub can do. They intentionally don't have an "algorithm" or any kind of content discoverability. Also the federation is limited to following users, on Lemmy you can follow topics and that causes all their posts and comments to federate.
Just today I heard Bluesky is making a Reddit alternative. I'm a bit worried they overtake Lemmy.
I think that was more because of Hexbear going down?