Sure, but the reason all the developers at my workplace use Slack is because we have to communicate with a lot of people who aren't developers.

Slack's good UX extends beyond the basic chat interface to configuration, administration, and initial signup. It's significantly easier for someone nontechnical to toss money at Slack to get a new private space than it is with IRC. The first page of "IRC hosting" search results on Google for me are mostly shell accounts; nothing turnkey and professional looking that a business person could/would use.