maybe the phone number requirement on signal? that would be a dealbreaker if any of them didn't have one

When me and my mates set this up, Signal was only available on phones, not desktops. It also required providing a phone number to a central authority, which some of us were not comfortable doing. With XMPP we got the choice of a large number of clients to pick from. Both the server and the clients were lightweight.

I just had a search maybe you can self host a Signal server, but I did not know that at the time. I wanted to self-host. So that was a reason too, but maybe (?) a false reason. The Signal self hosting situation may be murky. My brief search found some claims that the official app does not support using other servers, and you need a customized app to do it. It might be more self host-able in theory than in practice. XMPP had multiple servers to pick from, and lots of clients.

All those things could balance more toward Signal if your priorities are different, tho.