Focusing on fewer crops did take a while to develop. It was an ongoing process for a long time.

Let's take Illinois for an example. It was settled a bit earlier than the more western parts of the Midwest. The first farmers (from 1800 - 1830) were focused on self-sufficiency. They planted corn but also had large gardens for vegetables, raised livestock, and supplemented that with quite a bit of hunting. In the 1830s technical advances like the John Deere plow, and reaping machines made larger farms a better choice. The development of railroads and the Mississippi river trade made getting cash crops to anywhere much easier. Thus began large single crop farms. But the changes did not happen overnight.