[jeena]

Why would you need to use Slack (or IRC) when there is

    https://rocket.chat/
    https://zulipchat.com/
    https://matrix.org/
    https://mattermost.com/

[Crinus]

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.