Maybe it has something to do with something the USA holds as dear.
Separation of church and state.
The Roman Catholic Church WAS the state in so many countries. It had power over kings, putting fear for their eternal souls into the people to the point where the church controlled everything. Even the pressure of famine and war would not seriously affect the church, since tithes (remember the fear) MUST supply the church first, and, likely in any war, BOTH SIDES were part of the same church, whose power crossed all national boundaries.
Mabe thats simple, but it sure explains why the RC church didnt fall.