Lemmy has relatively poor automation and moderation tools compared to Reddit. There is no automatic moderator, for example, and you also don't have usability tools and things that other people have coded, unlike if you were to use Reddit today, or some years ago in its heyday. Beehaw defederated from lemmy.world because the tools at the time meant that anyone can sign up at .world, and post to beehaw, bypassing their registration restrictions.
Federation can also be a bit of a headache to get around if you're a layman. Do you need to go to all of the instances, or risk losing out on content that is hosted on other places? How do you sign up to it, etc. It's a good bit more complicated, and less intuitive compared to a centralised service like Reddit.
The user base also isn't big enough. Lemmy doesn't quite have the critical mass of users to make it into the mainstream, and put it in more direct competition with a place like Reddit. Even busy communities are really quiet compared to a relatively active thread. This community is arguably only as active as it seems to be because of the bot posts.
Finally, Lemmy is still pretty young, and has a fair few teething issues. It wasn't all that long ago that servers that updated to 0.18 couldn't see any posts from older servers, because they didn't Federate properly, and not all that long before that where Lemmy's UI had some serious glitches, such as by seeming to signing you in as someone else's account. People might want something that's a bit more stable, rather than moving over for sure.
It's probably part of why a fair few Subreddits moved to Discord, since it doesn't have a few of those issues, and is relatively mature.
