They come from all around the world. The US is all about diversity. We even show it through what we wear. The commercial world is just getting at that. Models these days aren't just blond hair, blue eyes.
People like to see something other than the same 'ol, same 'ol.
There was a time, no too long ago, when Caucasians completely dominated. They didn't want to accept them. Finally, they were more or less forced to accept minorities. This country has grown a lot from that day. But, we still have much room for improvement.