The way I understand from what I’ve seen and read is that these are mostly people who were banned, shunned or chased away from other platforms. While Reddit can ban you, you can just spin up an instance of Lemmy, Mastodon or whatever yourself and play by your own rules.

Interestingly, on Reddit there were tons of trans related posts and comments, but what I see here (and other fedi platforms) is trans people posting whatever interests them, and not about being trans. It’s like they can just be themselves without people constantly talking about or questioning their identity.