I haven't used a tool like Slack that I have so strongly integrated into my tech ecosystem. It seamlessly fit in with everything I use and is always up on my computer.
Non-technical users that are unable to use IRC require more handholding than other users. I'm not capable of providing that via a chat client.
There's only so much a bot can do, and the presentation of IRC varies wildly between clients. Slack normalizes all of that, which for casual users is huge.
Clearly it's drawn a lot of inspiration from IRC, and rightly so, but it's measurably better in a number of significant regards.
There's still a place for IRC and XMPP, but where Slack is a good fit, it's a great fit.
YMMV. Thanks
While I also like the polished UX, I'd argue that the only real value that slack adds on top of IRC is the UI/X and integrated history - the latter being something that IRC just plain lacks.