As a subsidiary question, why did they never domesticate bison? I imagine that they would have made perfectly serviceable beasts of burden for establishing farming on some of the most fertile land in the world.
hi! there's lots of room for more info on this, but fyi, both topics have been touched on before; check out these posts
Before the Europeans reached the Americas, did the American natives practice agriculture? - information on the plains starting at this comment
How did the Plains Native Americans live before the arrival of horses in North America?
Were there many large agricultural communities in North America between around 1200 and 1492?
re domesticating bison