Question

most northeastern could grow a wide variety of crops while Great Plains farmers were often limited to growing only a few varieties of crop, such as wheat. Why?

Answer 1

You get more extreme weather when you go to the center of the continent, far away from the moderating effect of the oceans. Hotter, dryer summers, and colder, wetter (snowier) winters. Hence the Great Plains were, before cultivation, grasslands, while the Northeast was covered in temperate forests. Not surprisingly, only the more hardy grass crops, like wheat, do well in the Great Plains.