It depends on the situation. Some were poor illiterate, immigrants. They came here, the best jobs they could get were in coal country, sometimes surrounded only by Protestants. Not being able to read or write letters, I think many just gave up trying to practice the faith, though they did not mix with the Protestants, at least in the first generation. Then, the babies born here, growing up with no Catholic culture, married a girl or boy from public school and were either converted by the heretic or their children were.

Poor, ignorant people are always the target of these groups, that's why they do so well in South America.