Probably because the few people who want to use alternatives to Slack are spread among these four alternatives instead of using one and sticking with it.
And honestly, of all the alternatives, IRC is by far the most popular one (and the other alternatives have IRC bridges) hence the most likely to stay around the longest.
Also the simplest to implement, which helps a lot too.
> it's good enough that it should be easily usable by developers.
Sure, but the reason all the developers at my workplace use Slack is because we have to communicate with a lot of people who aren't developers.
Slack's good UX extends beyond the basic chat interface to configuration, administration, and initial signup. It's significantly easier for someone nontechnical to toss money at Slack to get a new private space than it is with IRC. The first page of "IRC hosting" search results on Google for me are mostly shell accounts; nothing turnkey and professional looking that a business person could/would use.