There's a book called "Bowling Alone" that explores a lot of ideas around this. Iirc the conclusion is two fold:

1. Historically women were largely responsible for community building. As they joined the workforce, they had less time to community build and so there became less community.

2. Technology allowing home entertainment. People can now stream movies instead of going to theaters. Play computer games instead of go to arcades. Check Facebook instead of call friends to catch up. Use a Keurig for convenient coffee instead of go to a cafe.