Because the person looking for help doesn't need a Jabber client, but would need an IRC client. I'm sure there are web-IRC clients available too, but that's besides still the point, because Qunu gives us way more flexibility and control over the entire process. In the end it's really not about the protocol we've used, unless, of course Jabber transports allow us to connect to other IM networks. That'd be a different ballgame altogether.
It seems like the simplest, most obvious thing to do would be to provide the IRC information for people who have an IRC client already, and to provide a Java or AJAX IRC client right there in the web page. I know Java clients already exist, and you can set one up which defaults to or is limited to a specific server/room.
All academic now, at least from our perspective. We've chosen Jabber, and it's serving us well so far.