Why were the Sioux the most powerful tribe on the Great Plains by 1860?

Compared to the coastal regions of the country, the Plains
were thinly populated.  Hunting, especially for buffalo, was the mainstay of the Pawnee
and Sioux, and the large range of that animal forced the tribes to be nomadic to some
degree, and limited their overall numbers.

So I would say
there are three major reasons why the Sioux were the most powerful tribe on the Plains
in 1860.  First, the region would not support other competitor tribes and three Sioux
had come to mostly dominate over the years.  Second, the trade in guns and ammunition
had reached the Montana territories, so the tribe was better armed than most had been
when contact with whites took place, and third, Montana and the Dakotas were some of the
last regions in the Plains to be settled and ultimately conquered, so you could say the
Sioux were one of the few tribes still largely intact and unaffected by either war or
disease.