One possible answer is that white Americans considered the Plains tribes as technologically inferior to whites. At a time when battles between nations were fought by thousands and tens of thousands of soldiers in armies for whom logistics must have been a nightmare just to keep them fed, the Plains tribes still fought with spears, maces, and bows...or so the conventional thinking goes. While the tribal practices of training individual warriors may have seemed backwards, American Indians in the Sioux, Pawnee, Comanche, and Apache nations were relatively quick to adopt the latest repeating firearms. The adherence to nomadic hunting and herding perhaps seemed an outdated way of life, but this lifestyle allowed Plains tribes to pick up certain technologies, use them away from the "oversight" of white Americans, and keep the ones they found useful while discarding the technologies that were not.
Another possible answer is the sedentary vs. nomadic structure. White Americans perhaps enjoyed many more technological advances because of their ability to raise and store large surpluses of food, tools, clothing, and shelter, among other things, as a result of being "land-tied". The tribes enjoyed a perpetual surplus, as being nomads allowed them to move where they needed to in any season, following the herds of wild game that provided diet staples of grass-fed protein. So long as they took only what was needed to eat and utilize at the moment and maybe prepare a small surplus to live through the winter, the tribes could be certain of having more than enough animals to hunt following the winter months.