If you would build a software for communities (for discussion in general), then you would add different features that don't make sense for Slack or Discord.
It's a huge market with a lot of distinct use cases, and therefore can support a lot of similar players that focus on different use cases.
More general communities could use features such as:
- Monetization directly in the software vs. sending users to a signup and payment form (better UX, higher conversion rate)
- Discoverability (e.g., a search function similar to Reddit)
- Better onboarding for communities: For some communities, it makes sense not to be public, but to require a short application (for whatever reason) - currently community creators much build a separate website and send users there, etc. This would be much better if it existed natively.
- More focus on conversations/discussions instead of productivity tools
Just to mention a few. Of course, for Slack and Discord, it doesn't make sense to implement these features because it would dilute the whole product.