Farmland isn’t like a machine that can keep going forever, crop after crop after crop. It needs to rest now and then and build up a little fertility, and very often, the crops that are grown after a “green manure” (like a crop of clover) has been grown and plowed in are of higher quality than crops grown on continuously cropped land. However, farmers were paid for crops and production, not fallow land, so they were damaging their land by trying to grow as many crops as possible. When they are paid to let the land rest, they have more of an incentive to let the land recover between crops. Also, in a purely market economy, a decrease in product is supposed to increase the price for the rest of the remaining product. I’m not sure if this is a valid reason for the farmers to allow the land to lie fallow, though, because they already charge less for some things than it took to grow (thanks to subsidies for staple crops).