One reason for this is that corn subsidies have created an excess supply of corn-based food derivatives (e.g. corn syrup) for years, driving down costs and creating an incentive to use cheap corn-based ingredients across the entire food supply.
Farming subsidies were originally created in the aftermath of the Great Depression. While they have their place, we probably over-subsidize today. Given the choices of widespread obesity or widespread starvation, I'd choose the former.