I don't use Slack, but perhaps people prefer its UX?
DISCLAIMER: I never used Slack.
Simple example: with Slack (or another hosted, web-based chat app), I can invite the non-technical head of sales to join it via e-mail, and they can. They literally cannot install an IRC client.
I can get similar results, but "I" isn't enough for a communication tool to be useful.
I just don't get it...
Or pay someone to do that and maintain it for you.