Imo it's because sites like reddit make communities too open. It's common knowledge that once a sub regularly makes it to r/all, it loses all identity and joins the vague soup of r/all content which everyone upvotes with no regard for the source.

A lot of people don't want one big page with all the biggest communities thrown together. They just want to follow what they like and nothing else.

That said, the chat room format of discord is a pretty awkward stand-in for a forum type of community.