Many have speculated on what exactly caused the "sense of decline" that came to American communities, religions, towns, and so on after World War II.
But isn't it obvious?
We got automobiles en masse, huge highways, car-heavy suburbs, etc -- and everything became a CHOICE.
When you live in a dense, walkable area with no car (whether it's an urban neighborhood or a rural village), there is a "vibe" on the street. You see people you know constantly, involuntarily, and get invited to things. You have to go to them, because it's not as if you're going to teleport six towns over for a big concert on Friday night -- you're going to be there, and whatever's going on is what's on offer.
At those events -- from Parish suppers to funeral Masses to hockey nights at the pub -- you're going to be engaged in a kind of "theatre" of culture, you'll be a representative of your place, your family, your faith, etc. And as you and all the other neighborhood people coalesce in this fashion, you will be REIFYING your sense of culture, identity, faith, and so on. You'll be proving it to each other, refining it, being a part of something.
The minute you add automobiles to this picture, all of that stuff is an "option" -- and it just so happens that culture has ever functioned in the manner we've known because it has been 100% non-optional, in your face, intensely local, etc. Automobiles disrupt this massively, and because of them, local pride, local Parishes, local accents, and basically all things that give us an identity and sense of collective purpose have died back and atrophied.
The saddest aspect of the US' cultural decline is the death of local pride. Midcentury historical societies flourished in small towns throughout the country, with the help of local gentry and profs from local colleges. The death of local pride is the death of broader republic.