To be fair, (as a Slack user) I'm not sure off-hand what Slack has that couldn't have been done on top of IRC. Almost everything could have been done with an IRC Server + Backend Services + Fancy Client model.
Problem: "Backend services" - how many of the Slack integrations don't have public APIs and Slack was only able to do it by being a single company? Good luck convincing people to set up the API keys for all of those services.
Why can't Slack run irc servers on their own equipment? The "integrations" would just be IRC bots running on private Slack servers, no?