WhatsApp is more sticky for mass Indian audience, other platforms don't have the same kind of audience WhatsApp does.
Even though I'm a Telegram fan myself, too many crypto scammers and phishers there. Discord is more genz, others find it difficult to use. Slack is more work oriented, plus its free version isn't suitable enough for large communities.
I've been running and managing various communities since a decade or so tbh. It's become much easier to sell to communities now, as long as there's value provided. Most people still don't get how to provide value first tbh, they still try to sell first. It's pull vs push.
Organic social reach is dying in favour of ads. Communities and messaging give you direct access to your consumer and let you have a deeper relationship with them.