From the comments

2) Lack of binding ties. Like it or not, we live in a world of voluntary association. Community is inversely proportional to that. Humans historically lived in family-groups, small tribes, etc with essentially involuntary membership.

I don’t see a way to bring that back, but to make community stick people have to have enough commonality to act as a glue. Faith is a good start, but obviously not enough.

This is where I falter a bit. Going back to my college experience, the binder was a common set of experiences, similar age, shared interests, etc. I don’t know how to scale that.