It's really hard to compete with platform stickiness for peer/interest groups. If you're already in Slack you're more likely to stay engaged with other communities on the platform. Ditto for folks on Reddit, WhatsApp, FB Groups, Discord, etc... otherwise you need a very strong anchor to pull people out of their existing routines/habits.
IMO that's why product support and creator-led learning communities have the best shot at doing well outside of Slack. They're more transactional than conversational. You get in because you're looking for answers or guidance, and you're more likely to go along with whatever you're told to use.