The problem with IRC is that the plugins aren't as discoverable and more of a pain in the neck to integrate. When you're not in channel on IRC, you don't have

The problem with IRC is that the plugins aren't as discoverable and more of a pain in the neck to integrate. When you're not in channel on IRC, you don't have access to anything that was said prior to joining the channel which means you don't even have any immediate history, or context to a conversation that is ongoing when you join. At least with Slack you have 10,000 messages of history, which is at least enough to give you the context of the discussion. Also while probably irrelevant on a technical level, it's just not as pretty and user friendly as Slack.