Why are the great plains grassy? Why so few trees?

Question

In most of the US east of the Mississippi, grassy fields, if left alone, become pine and eventually hardwood forest in a few decades. Why do forests not intrude into the plains states? I suspect it has to do with pH, rainfall amounts, natural disturbances such as grazing and fire, but I don't know.

Answer 1

Yes, most of the plains is grazing land for cattle...and for growing grains.....so you will have something to eat.

Answer 2

Rainfall, or more precisely, lack of rainfall is the primary reason. Fires also plays a role because there's more lightning strikes out in tornado alley.