The official answer: https://matrix.org/docs/guides/faq.h...atrix-and-xmpp

Personally, I'm not convinced that there really is that big a difference. I have no experience with XMPP MUC (or whatever it's called -- using XMPP in a group- or room-oriented fashion). But that may be an indication of part of the difference -- I've never encountered a means of using XMPP for group- or room-oriented communications that is as easy as matrix is...