Lemmy naturally concentrates unconnected users with similar interests thanks to reddit-style communities. Mastodon follows the Twitter style where you have to find and follow individual users to get their microblog content, and its harder to isolate certain topics or interests except across the entire service via hashtags. Individual users on their own are very uninteresting and bland.
Lemmy has fewer users but they as a whole generate more active content than Mastodon does thanks to community specialization, since the Twitter style posts require some critical mass of users following to generate interesting discussion (something that basically never happens unless you’re already a celebrity)

So many posts perfectly summarising why I’ve always preferred the reddit format over twitter. On one you follow topics, on the other you follow people. I prefer to hear a wide range of views on one topic rather than one persons views on different topics.

Just guessing here, but Lemmy is generally content focused, where it feels like mastodon and twitter have more of a focus on the interaction between users. This would mean that Mastodon needs a lot of active users to function, where a lemmy community can be largely carried by just a few really active posters.

The main factor is discoverability.

There are no shortage of creative or funny people on Mastodon, however, Mastodon’s feed algorithm do not allow them to be discovered unless you happen to stumble upon them by happenstance, whereas it is quite easy to be seen on Lemmy by posting good content: it’s rare when I don’t get any upvotes or downvotes on a comment here, and good replies are fairly common, so the interaction quality here is generally higher.

Mastodon is just a bunch of news articles and people talking like robots. I try to engage and there’s fucking nothing I care about. Anything actually interesting is like half a thought. Like they started talking about a topic but didn’t get to the point before they decided to hit post. Posts from popular accounts talk about electoral politics in a weird clipped manner like a newspaper but even more boring.

Mastodon feels like a torrent of random unrelated comments drowing out anything that might be interesting. I tried it, I don’t see any value in it. Even for following friends it’s unusable, there is the one that posts three times a day and the one that posts once every three weeks, there is no way to ever see one of his posts, unless I specifically go to his profile to look. I’ve given up on Mastodon.

I personally would rather follow topics than people. I don’t know or care what the founder of Adobe had for breakfast. I like the idea of community aggregate voting to drive an interesting feed. Maybe Mastodon can do that better than I know because I only gave it a few days… but I was nowhere near what I wanted after a few days where Lemmy was good from day 0.