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.