This article is comparing apples to oranges. IRC is a protocol, Slack is a client/app that uses protocols (one of which used to be IRC). Many IRC clients, although beloved by some users, are rough around the edges.

In my opinion, Slack has "won" over other clients because of UX: it's extremely easy to sign-up, understand, and use productively. Plus, even large orgs (ours included), can use the free version if they're OK with having chat history disappear after a few days/weeks.