It keep being surprised by how much a fresh coat of CSS and a mobile app can reinvigorate these old ideas. Who knew how important adding gifs and emoticons would be?
I'd say history access/logging by default, easy sign-up, easy management/configuration are all way more important reasons. Someone very likely could have gotten all that working as a good product based on IRC, but they didn't. (I use IRC a lot more than Slack, but my requirements are not the same as those of a company adopting Slack)
It keep being surprised by how much a fresh coat of CSS and a mobile app can reinvigorate these old ideas. Who knew how important adding gifs and emoticons would be?