Can you give precise examples of the problems? As someone who tried to use XMPP in the past, I found it to be far worse than IRC in terms of connectivity and how things worked. Matrix.org couldn't really have used IRC as a protocol because IRC isn't a very extensible protocol. I've been using Matrix.org for several years now, and it's much better than my experiences with IRC and XMPP (though IRC is still quite usable, and I do still use it regularly).