An 2002, interesting examination of enduring levels of inequality in the United States and research on the history of enduring levels of inequality and research on why some regions experience limited social mobility and economic underdevelopment—than others.
Economists argue that the answer lies in "the earliest history of each country’s settlement. The political and social institutions put in place then tended to perpetuate the status quo. They concluded that societies that began “with extreme inequality tended to adopt institutions that served to advantage members of the elite and hamper social mobility.”
The result, they believe, is "economic underdevelopment over the long run."