When Italy had been Catholic for about 500 years, it was overrun by barbarians and sank into the dark ages.
Maybe you should re-check your history :)
Actually, one of the differences between Catholicism and other religions in the New World is that the Church catechized and baptized the indigenous peoples, and then Spaniards married them. The English did not; furthermore, there weren't a lot of them indigenous peoples in the US, compared to the number of tribal peoples in Latin America, so the Indian population would probably never have had the weight that it did in Latin American.
The result was that the Church had to deal with a large number of very primitive indigenous populations throughout Latin America. This meant not only merely preaching the Gospel, but in many ways adapting these people to the modern cultures that had grown up around them.
I think, had the changes of the '60s and '70s not occurred, we'd be seeing a very different Latin America. For one thing, prior to VatII, the urban wealthy were being brought by movements like Opus Dei and others to realize their social responsibility to educate and aid - but once the Communists got in, that was one of the first groups they attacked. And they have continued to do so since then. I always think of the poor woman who was buried alive by Communist guerrillas in the early 2000's (I don't reacll the date)- she was from a wealthy family but had spent most of her time starting schools for poor, mostly Indian children, etc. And the "revolutionaries" couldn't stand that. The influence of Marxism, probably by way of Mexico or as a hangover from the "revolutionary" movements of the 1920s, revived once the Church weakened after Vatican II, and the Catholic Church and good Catholics were one of its prime targets.
Europe after the fall of Rome was essentially in the same condition. Rome was gone and the barbarian peoples (many of whom, btw, were not orthodox Catholics, but Arians) were way more numerous than the cultivated remainder from Rome. In Western Europe, there was an enormous amount of Catholic intellectual activity in Spain, particularly Sevilla, but that all stopped dead with the Muslim invasions.
As for personal piety, Latin Americans used to be very good at it, if a lot more dramatic than most North Americans like. It wasn't easy, and they weren't educated people who had a lot of resources. But once Vatican II killed personal piety, killed things like the necessity for Confession, the need to baptize your children, the importance of marrying in the Church, etc., the smoking wick was quenched. Being a well instructed Catholic doesn't necessarily mean you will be living a good life - but it does mean you'll always know you're doing wrong, and therefore you know that you should repent and get your life straightened up. Some people did, some people didn't; but everybody knew the message.
That was all swept away with the "social gospel" promulgated after Vatican II. Suddenly the poor were no longer moral beings with individual responsibilities before the Lord, but simply "the poor," a project for social workers and "revolutionaries." And now we're seeing the result of that.