a) Slack _clearly_ offers a lot of meaningful functionality over-and-above IRC channels. "Searching" - and, implicitly, persistence - is so fundamental to the offering that it's (apocryphally) part of the acronymic name. Threading, bot support, and channel discovery are all useful features. Sure, all of those things _can_ be implemented on an IRC server, but they're not out-of-the-box.
b) Setting up and supporting an IRC server is non-trivial for a non-technical person. Sure, it's easy to you and me - but any system that can allow customers to get access to that functionality _without_ needing a dedicated I.T. team is going to be more attractive to decision-makers.
