Because you could swap off irccloud at any time, to another IRC services provider or self-hosting, and not lose touch with the people you're chatting with, because the network layer is independent from your interface (irccloud).

By contrast, if you want to switch off Slack, can you still talk to people who stay on Slack, or do you have to get them to switch too?

I like IRC over Slack for the same reason I like email over Slack: I can just use my client with the interface I like, and the network takes care of getting my message to someone using a different interface, that they like. It doesn't require we compromise on a shared, fixed interface, a la Slack.