I've been engaging with Lemmy more than Mastodon. Lemmy allows for more interactions through discussions. To me, it seems like Mastodon is slower to get interactions (for context, I have accounts on three different instances and used to post dark/gothic/satirical/surrealist poetry written by me daily, but I haven't posted in days because no one seems to be really engaging with it). Mastodon has a lot of potential, but I think few are really committed to stick to the fediverse and all its potential.
As for why people don't come back, maybe they're confused about which instance to use since there are thousands of different instances for different purposes, I'm not exactly sure.