Thats because the U.S. became more economically dispersed from the industrial NE and Midwest to the South and the West. Kinda difficult to have "community" when you live in the land of Office Parks and Applebees.
On the other hand, the "Megachurches" are doing very well. I think the RCs main problem is that it is run by incompetants (the late PJPII being an exception) and hasn't been "on message" consistently since the Second Vatican Council.
Hence the auditing of Catholic Charities.