Because it's like IRC, but way easier for the average non-technical user to set up and use.

I have ran my own private IRC server before and frankly, it's a pain. There are lots of solutions to common problems (like using IRC bouncers) but it is just complicated enough to make products like Slack a superior user experience.

It is pretty easy to get grandma up and running on Slack. It is much more difficult to get her to use IRC.