IRC is quite capable, but Slack is like IRC without all the rough edges and with a nice API.

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.