The "Farm Bill" contains many provisions which drive the overproduction of commodity crops like corn, and create a disincentive to produce healthy food such as fruits and vegetables. This is one reason why so much junk food is comprised of predominantly corn by-products: corn is cheap and easy to chemically alter for many purposes. Unfortunately, there is only so much crop land in the US, and when there is so much money to be made raising corn (due to the subsidies the Farm Bill provides), farmers aren't very motivated to produce much of anything else.
The solution is simple: reduce or eliminate subsidies for commodities like corn and wheat. Introduce or increase subsidies on those foods which are truly nutritious and good for human consumption: namely fruits and vegetables.