When I joined Hypothesis, my new employer, I wondered: "Why are we using IRC instead of Slack?"
It boiled down to three reasons. First: inertia. The company had established an IRC culture before Slack existed. Second: openness. We're an open source nonprofit, so we operate in public mode as much as we can, and Slack doesn't yet support that mode natively. Third: skepticism. Sure, Slack is the flavor of the month, and all the cool kids use it, but is that really the right reason to switch from standard IRC to a slightly-improved-but-proprietary implementation of IRC?
We continue to waffle on the subject. I expect we'll land in the Slack camp eventually because, to be honest, we do want to play with the cool kids on their home turf. Meanwhile, though, we're enjoying the same kind of virtual water-cooler effect in IRC that we'd be enjoying in Slack.