I've played the USA in two GCs, and best I can figure out is that the USA is hardcoded to get strong immigration throughout the game. The reason Okinawa is the place to be is that it is the geographically closest place which is considered US territory to the emigrating nationalities. Buy Bermuda from the Brits and the same thing happens--I had 30+ farmers in the British (well, American) West Indies. They just come pouring in.
Immigration does slow down in the later game. I believe that two factors play a major role 1) conflicts in the Old World--more wars, more immigration, and 2) social/political changes in the the original countries (they reduce emigration to US).