I've never understood why slack is so famous and got so much traction when other alternatives existed way before it, like hipchat
In a few more words, the product appears to have been designed with user needs in focus more than the others, and the end product is closer to solving them. The article reveals some of the ways they've achieved that. Furthermore, it goes beyond simply meeting user needs into a superlative user experience that also exceeds expectations. It's not too complicated: a big part of their success is that they have created a better product in the eyes of users.
There were interviews from last year where he talked about slowly converting teams from free to paid.
I use slack for a side project with 15 or so people involved and while we're using the free version, it is on the list of things to pay for once we have some sort of revenue.