The No. 1 culprit behind our overreliance on corn is the federal farm subsidy program. While subsidies are not categorically bad, they become a problem if they leave farmers with little choice but to focus on a few crops. The proposed farm bill now before Congress would make some progress by ending direct payments to farmers for certain commodities (most notably corn) in favor of expanded crop insurance. Even with that critical change, a floor price (below which farmers receive payments from the government) and a more robust crop insurance program for certain commodities will still mean that farmers narrowly concentrate on corn and soybeans in the Midwest.
While this system clearly favors those interests that benefit from an oversupply of cheap corn (fertilizer and pesticide providers, feedlots and food and ethanol producers), it is not good for taxpayers, our food system or the environment.
And we need to change this system because over-reliance on any single crop is a bad idea for an economy, in which diversity is a sensible risk-averse strategy, and it's a bad idea for consumers, for whom a varied diet is more nutritious and healthy.