Mastodon is basically just a bunch of guilty liberals who feel bad that their baby had became a nazi, but are not willing to take the steps necessary to grow from there. Lemmy comes from redditors who saw the platform as always being full of overt displays of nazism and thus were able to appropriately grow from the experience and free themselves from its influence.

When you look back at your reddit days you are ashamed, because you now know why it was wrong. When mastodon users look back at twitter they reminisce with fondness of their times hanging out with war criminals and retweeting right wing sources that agreed with them.