Bison territory in the United States when we got here was not limited to the plains, in fact they still are in Northern Mexico which is but a small part of the real story.
In the La Brea Tarpits in Southern California of all the animals they have recovered bones for Amrican Bison is number one and orders of magnitude more than any other animals they have discovered.
Now i know there will be snarky remarks about Bisons mental capacity but they are ill formed. They have never been domesticated as Cows and other animals have and they are fearless. They calve in snow and if a predator is dumb enough to tray and take down say a calf they form a circle around it and then the action starts.
You may have heard of Buffalo grass that grew whereever they roamed, sometimes high enough that a man on horseback could not be seen. Buffalos provide an invaluable service as they eat, pee, poop and make baby buffalos. Their hooves are not like cattle and they till the soil as the run rather than flatten it out as other animals like horses and Cattle do
They regenerate the land and eat invasive plant species
I have a place on the south rim directly across the canyon from this and I can tell you for a fact that there is not enough grazing feed land to support Bison. There is very little that will grow under all the Juniper and Pinyon Pines. Sparse Manzanita, Sage, and some Cacti. Let alone that most of the terrain is fit for only Deer and Dall sheep. Even the Elk and Pronghorn Antelope there prefer not to navigate most of it, too steep.