What Trident says is pretty much true, the US hasn't had nearly as many issues surrounding the rise of fundamentalism and general integration from within its own boarders as many European nations.
With modern travel it's becoming less of a factor all the time, but still the US has a big advantage of having two huge oceans separating us from the rest of the world and bordering cultures that might conflict (with the exception of Mexico) unlike Europe has. I'm sure that has a large part in it.