I would say the main thing that is difference in these countries is lack of racial/economic diversity. Whether anyone wants to admit it or not, racial tension is highly prevalent in the US and is a major component to most crime that you see. And sadly, that's just the facts of life. Different races just don't get along, and it's been that way since the beginning of time. And it's going to stay the way for long past the time we are dead.
It doesn't take a rocket scientist to notice that Sweden and Switzerland (high gun ownership/low crime rate countries) have a lot of rich white people living there and not much racial diversity. And rich white people don't go around shooting each other all that often.