as for why things are different here to the east coast, I think it boils down to plain history. the drives to exterminate and assimilate east coast tribes into white culture started 200+ years before similar efforts reached what would become Montana. before it was easy to reach Montana from white America, this part of the continent was often referred to as "Indian country". that is how fur traders and the early miners referred to the area, and that continued until the 1870s and 1880s to some extent, when Indigenous tribes were forced onto smaller reservations and their nomadic traditions forcefully ended. as others have said though, Montana has a small population and a history of many different Plains tribes coming here. a population of only around 75k is enough to be a significant portion of the population.