Well first of all there was no "it" when it came to IRC 15 years ago. There were many huge servers that I never visited like Efnet, Quakenet, dalnet and more.
Me and my little group of hackers would also host our own, and link with other groups around the world.
I think it boils down to accessibility. Even back when I started certain people just couldn't handle the terminal clients so they used Xchat or Mirc. In school in Sweden IRC was actually often pronounced "mirck" because people would refer to the mirc client.
So when web chat came along, with mobile apps too, it just reaches a much wider audience.
So the ones who are left on IRC are old farts, and terminal junkies.
