With rents going up and switching jobs more frequently than past generations it becomes harder to stuck around in one place long enough for strong communities to form.
The reality is that we just don't talk to each other much. If you want to look for the decline of one institution that provided some degree of community cohesion, then the answer is going to be pretty unpopular: we don't go to church anymore.
Strictly true, but if people aren't strongly compelled to show up somewhere, they aren't apt to. Historically, the "fear of God" ensured that people went to church, effectively forcing them to gather, meet each other, so on and so forth.
The only thing I see that comes close to that these days is youth sports leagues, where parents have a fear of failing their children if not heavily involved in such activities. While this does seem to establish some kind of community amongst parents of children of similar ages, it does not seem to expand out into parents with children of dissimilar ages even within the same sport, let alone an even larger community. Which is not terribly surprising as there is not much reason for someone unrelated to a participating child to show up.
I might even go as far as to say that because parents get stuck in those narrowly isolated communities, it contributes a fracturing to any larger community that might have otherwise been able to form. It is difficult to have a general community of only 20-somethings and the elderly with little in-between.
https://www.ft.com/content/96cb501d-b188-4e50-af21-ca7115878...
Frankly, it's hard for me to imagine what could possibly create a community in my neighborhood today. Half the properties here are AirBNB party houses. The average tenure of a neighbor who actually lives in their own property feels like maybe three years. Hard to make any kind of lasting connections, with or without a church.