They were packed, I think, with the children of immigrants and those older grandchildren who did not question anything.  The end of Vatican II happened to coincide, in this country, with a lot of questioning of authority: Vietnam, middle class women's roles, the sexual double standard, women's rights, gay rights, etc.  I think that it was a fluke that Vatican II ended just as the more rebellious sixties began.  

To put it in, perhaps, superficial but musical terms, it was a time when, thanks to the coming of age of the first Boomers, it finally became apparent that "Swing and Sway with Sammy Kaye" was indeed going to be replaced by "Rock and Roll is Here to Stay."