I think it's actually an issue of perception. When I hear Jabber or XMPP, I think of one-to-one chat and when I hear IRC I definitely think of group chat. In general, IRC clients seem more tailored towards groups.
That said, there's definitely room for both protocols and I could see supporting XMPP as well at some point.
I don't really think that's true. Look at Jabber.org which is probably the biggest jabber network out there and it seems they're scaling pretty well.
Edit: Also XMPP supports UTF-8 for everyhting out of the box and as far as I know it's not really possible with IRC (or did that change over the last years?). E.g. nicks with non-ascii chars are no problem to have..
XMPP MUC also has the problem that the largest XMPP client, Google Talk in Gmail, afaik does not support it. It handles multi-user chats via some proprietary method that only works for Gtalk clients.
That said, there's definitely room for both protocols and I could see supporting XMPP as well at some point.