It is difficult to say for sure. There are probably thousands of reasons. But I would assume the answer has something to do with Protestantism, to be specific. If you aren't a Catholic, chances are that you're Protestant, and the bulk of the Protestant population is American. They have different approaches on how they choose to practice Christianity, specifically "preaching", or "witnessing". They believe it is their mission to spread their views and the word of God to not only score points in Heaven but also so "save souls" and lead them to Christ. As you know, the basis of Christianity, that if you accept or believe in Jesus, you "will not perish, but will have eternal life." Sounds beautiful, eloquent, poetic, and philosophically benevolent, right? Only... that's IF you accept it. So, Buddhists, Muslims, Jews, and many other religions, as well as non-religious people seemed to be ineligible for eternal paradise.
Though, more Liberal Christians seem to think that it isn't, nor should be, that simple. And that, as long as you are good and follow your conscience, it wouldn't matter, which is what I respect.
But yeah, to shortly answer your question, I would say, geography, sociology, American culture of being loud and "out there", as well as Protestantism, are prime factors. Plus, Europeans are generally more secular and religion seems to be dwindling in numbers there, compared to the U.S. So, it wouldn't surprise me if people are religiously "to themselves" or more reserved. I would certainly say that Europe has always been ahead of the game when it comes to tolerance, acceptance, civil rights, and other social issues relatively more than Americans. Protestant Americans aren't very accepting of many radical views, like homosexuality, hedonism, liberal "free, chill" lifestyles, Atheism, Socialism, Communism, and many other things that would be more Conservative, as opposed to Liberal Europe.
Now that's my sociological perspective, but it's much more complicated than that, and there are no doubt thousands of other reasons out there. I would in part say that our educational system isn't exactly "up to date" in quality and funding as Europe. We aren't the most open-minded, freethinking, or ACCEPTING critical thinkers.
And science isn't the most popular, liked, appreciated, or understood field here, which also probably plays a role. Heck, almost half of Americans don't believe in evolution, the big bang, and a quarter of us ACTUALLY thinks that the SUN REVOLVES AROUND THE EARTH. So half of Americans actually have an adequate appreciation and understanding of science, while a quarter is lacking the fundamentals, with the bottom quarter living in total ignorance. An American lover of science, it heartbreaks me. Now that's not to say that Big Bang/Evolution/Geocentrism is mutually exclusive of a God existing, but it probably has something to do with religious extremism here.
Though I would say that most of whom you are talking about is probably southerners, "Bible-Belters", north westerners, and conservative country hicks who aren't as well educated or liberal as the inner city fellas.
I'm sure, the First Amendment of our Constitution that encourages and permits free speech is emphasized more here. I believe Europe has more limited speech, etc. We can literally say, believe, and express ourselves however we want as long as it isn't violent, or creating public havoc, threatening national security, etc. I believe it's limited in Europe. For example, you can be racist and express it as long as you aren't placing anyone in danger and not be penalized by the government, Mein Kampf is legal here, etc. It's the "Free Market of Ideas", even if the ideas are ignorant, sadly. So in a way, we are encouraged to be "out there" with our views and shove them down each other's throats.
It's the American Way.
But this is GENERALLY speaking, of course.