What's wrong with the UX of IRC?
* No user accounts, except via a baffling NickServ process involving things like 'ghosting'
* Poor abuse tools, e.g. accidentally IP-banning web IRC gateways.
* No searchable history
* No scrollback / visibility of messages that arrived while you were offline
* No support for uploading images or files.
* No sharing of a single identity / session across multiple clients (e.g. phone+pc+laptop)
As a veteran IRC user I know you can mitigate these problems by getting a shell account on an always-on unix server and running Irssi in Screen over SSH with logging, automated NickServ login, a bot to provide !seen and !tell, sharing images via Imgur, sharing files by uploading them to my personal website, and so on.
Slack has simply looked at the big collection of bodges I just described and made them native features. And you don't even need to know to use Ctrl+Alt+2 to change between channels like you do with ssh+screen+irssi.