Earlier quoted context omitted.
The difference is people like using it. UX counts for a lot. The massive amount of integrations for it helps a lot too. Their goal, from what I've read, is to be a giant log for everything in your company, not just a chatroom service.
"Their goal, from what I've read, is to be a giant log for everything in your company..." Why would you ever want to entrust a log of everything in your company to a third party service? Honestly curious.
Slack has said before that they're going to release a self hosted version at some point, so they have to know this is true. I think it makes sense for them to focus on the larger market (personal use and the SMB sector) for now though, even though the enterprise market will really make them money, because it's a much lower pressure space where they can afford to experiment.