Well IRC is able to go down just like Slack, especially if you're using a smaller/private network (I'm not going to run my organisation's 'Slack' on Freenode).

Again, as these things are, the tech has nothing to do with it. Hipchat 'lost' to Slack while being more or less identical (from a raw features/technical point of view). Hell, Slack could be IRC for all I know, but what makes people use Slack is more than what protocol it uses underneath.