Both of those probably also have a small fraction of the users of Signal
Change has to start somewhere, just like Lemmy/Piefed has a fraction of the users of Reddit :)
DeltaChat is cool but it lacks forward secrecy for now and the foreseeable future
That may or may not be an issue depending on the threat model of the individual. Someone that suspects a state actor may be surveilling them? They'd likely want something more secure (though I personally wouldn't trust Signal in that scenario either due to their centralized nature, which makes them an easy target for state actors).
For someone just wanting a group chat with friends/family that they don't want being used for AI training or sold to ad companies to build a profile on them? Deltachat is more than secure enough, and certainly better than the corporate owned options they likely already use.
and XMPP is a newbie-excluding mess (for example, encrypted group chats require every user to authenticate every other user)
I haven't needed to authenticate anything manually in my experience in the XMPP apps I've used. I just enable OMEMO encryption in the chat I'm in, and as long as the other participants also have that option enabled, it usually just works, and they can read my messages. Its certainly worked better and more smoothly than Matrix's encrypted chats, which consistently had issues with messages not decrypting.