It's weird because the reservations are supposed to be sovereign nations, though no one outside the US recognizes them as such. But in effect the people are basically wards of the Bureau of Land Management. Regulations are so layered and onerous, that they can't be built up economically (except for casinos, natch), so have always remained poor with no chance of getting out of it.
Even basic groceries are bought off the res, which sadly, also sell beer and whatnot so there's no way of trying to regulate that problem.