Well, that's kind of the point. Slack has built a moat. IRC is libre and free, but also can get far more arcane than the RFC suggests and to get something like Slack's feature set, you have to either build or strap together a lot of moving parts. Discourse is targeted at gamers and is not good for businesses. Ryver is way behind. MS' offering is laughable. Slack also has exactly the kind of differentiating features that don't particularly matter to ICs, but which corporate values highly—like auditing, retention policy, and permissions stuff. There's a specific subset of "good product" which has the word-of-mouth marketing result that Slack gets, and that's not the same as being "good" in a vacuum.

People don't use the alternatives to Slack because they're not as good as Slack, on various axes, is the short version. Slack has made themself hard to compete with.