The US has had, in its history, a large-scale immigration from just about every region of the world, and most of them have interbred. The result is a population with lots of outbreeding depression and heterosis, leading to a much wider variation in intelligence and other abilities than anywhere in the world. The ultimate outcome of that is a lower average intelligence in the US than in other countries, due to outbreeding depression, counterbalanced by a small number of exceptional people, who got lucky and benefited from heterosis.