There are two reasons. The first is that just about everyone is white, with a homogeneous culture.
The second is that just about everyone is middle-class. In the U.S. the wealthy are overwhelmingly parasites who have used government to suck wealth from everyone else, and as for the poor, they have plenty of money for alcohol and drugs and big-screen TVs...as long as someone else pays for it.
The middle-class is the zenith of Western civilization. When it goes we all go, rich and poor alike.
By the way, when you look at just the white homicide rate in the U.S. by firearms, it's lower than Holland's.