There are a number of ways that we can explain this. These factors combine to cause the phenomenon that you describe.
- History. Historically, America was a land of opportunity mainly for white people. Many of these opportunities came at the expense of non-whites. The “free” land that whites enjoyed was, of course, taken from the Native Americans. This was the beginning of their problems. They were put on reservations and not really given any way to participate in the American dream.
- Culture. Having been put on reservations (and wanting to maintain their culture at least to some degree), Native Americans have not assimilated well into American culture. We must remember that it was not much more than 100 years ago that their culture was largely that of hunter-gatherers. Since they have mostly not been aggressively assimilated, it makes sense that their culture will still hold values that are not necessarily useful to modern life and the expectations of a modern economy.
- Continuing mistreatment. The US government has typically not been eager to put a great deal of money into helping Native Americans. This is partly responsible for the fact that their communities remain poor and their educational systems somewhat substandard. This limits their opportunities.
It is hard to know how much relative weight to put on each of these factors, but all contribute to continuing Native American problems in the land of opportunity.