I remember one theory had the excellence of Jamaicans secondary to the fact that the most intelligent ones figured that they would jump ship in Jamaica before getting to the States!
There are a number of reasons why Caribbean people have done better than black Americans, at least in New York. John Perazzo covers some of the history of the black West Indian in America in his book pretty well. Actually, right now I did read a stat that said black immigrants are now outpacing some white people in this area in terms of income because many are self made people who own their own businesses. Even when Caribbean people in general, not just black, started immigrating here they were very self employed minded people. The most famous example is Alexander Hamilton but Perazzo's book only covers black people.
Black caribes come to this country never having known US racism and the history of racism so they have a very new and fresh perspective. Many black immigrants in general don't have the same hangups. They've not been exposed to the mental baggage that comes with hearing the same lies told over and over by civil rights activists.
I could go on but I'll just say that when my mom first came to this country she worked very hard but she said she avoided watching television. She said watching what was going on in the south made her depressed.