The interaction of three factors maintains the Prairie as a grassland:

Drought: The periodic lack of rainfall is a very important factor in preventing the growth of trees in prairies. A drought is when the prairie receives no more than 10 inches of rain per year. In the picture below, you can see how a few trees are tenuously rooted in slight depressions where what little moisture there is gets concentrated.

Grazing: Continuous grazing by bison, prairie dogs, and numerous other species, keeps the grass short. In a feedback cycle, the grazing also stimulates new shoots to grow which the grazing animals depend upon. As bison graze, their hoof-action also sows new seeds into the earth which helps to promote new vegetation.

Fire: Every three to seven years, any given area of tall grass prairie is likely to burn due to fires started naturally by lightning. This too prevents long-lived species like trees from becoming established.