The advantage of IRC over nearly any other chat method is it is still one of the best One to Many chat options out there. And just about the only option that I know of that allows fairly random people to join a channel and have it moderated. I'm not the biggest fan of putting random people on my MSN or something to chat with about random things, but IRC fits perfectly for that problem.
The reason why no one uses it is because no one is using it. If a large enough group of people made an effort to log on and chat when people are there, then you would see the channel liven up.