They're just too stupid, I suppose.
Seriously, I oversimplified J. Diamond's argument, in order to express it a few words. You seem to argue for an "IQ" determinism. This seems silly given the rise and fall of civilizations over time. And since America is the most mongrel of civilizations, it's hard to believe that only the most intelligent emigrated here. The reason for the success of our culture is our embrace of freedom in economics, politics, and other social spheres, not an "average" ability to match 3-D projections.