One of the most valuable things about Slack is the ecosystem: apps, API support, etc. If you need to receive notifications from external apps (like PageDuty or Incident.io or something like that), good luck expecting them having a setup for your own version of the app. Yeah, some of them provide webhooks (not all of them), but in the end you have to maintain that too...
Also though, I feel like being attached to Confluence helped it because there is a lot less competition in the world of documentation wikis than there is in task management.