'Today's kids' use MSN more than any other IRC/IM client because it's basically forced upon them by Microsoft. They don't have to download anything, they don't have to learn how to use it, all they do is sign up and they're set.
IRC clients tend to require a basic knowledge in IRC or the client itself and other IM clients like Yahoo! Messenger have to be downloaded.
In my opinion it has nothing to do with functionality, but convenience.
