Answer:
Geography dictates history. For example, the Nez Perce Indians of Idaho-Oregon had great rivers like the Columbia, etc. So the Nez Perce would eat a lot of fish. They also had the horse culture because geographical features did not prevent horses to make it to Nez Perce Country. Because the Great Plains were in the West of Nez Perce Country, they could go there and hunt and kill buffalo. So, in the end, if they could not survive on the fishing culture, they could make it with the Buffalo Culture.
Compared to the Nez Perce Indians, other Native Culture of the Plains did not have the fish and sometimes the buffalo did not always wander in Sioux Indian Country and then the Sioux would struggle unless they could kill a lot of deer and so on. But if they had the fish culture, they could make up for the lack of buffalo.
Because the Great Plains had rivers, various Native American tribes would camp along these rivers while they were following the buffalo. So again, the geography of the area provided this availability of food and places to stay and feed and water their horses.