The audience for the Internet changed. IRC was great when everyone on the 'net was tech savvy, but look at what happened when people were able to pick between AOL and direct dial-up connections. Facebook is so simple my grandmother could figure it out, but even with decent clients like Colloquy, I wouldn't expect her to find her way around IRC. As for the Enterprise, HipChat and the like, especially when served via SaaS, offers more than just a chat server - server management, auditing, easy user setup, integrations to other tools.

For what it's worth, I agree that it's a shame that we are barely better off than we were 20 years ago, but I'm willing to accept that if it means widening the audience of the Internet for everyone. You shouldn't need a CS degree to surf the world's information.