It has a lot to do with the federal government badly skewing the market with subsidies. Look:
If the government is going to game the system via subsidies, why not subsidize the production of fresh fruits and vegetables, to make healthy food more accessible to poorer families? Government also supports the market by buying up surplus meat and dairy products, and distributing them to public schoolkids in their lunches. As the Physicians Committee for Responsible Medicine (whence the chart above) points out, the government is not required to buy fresh fruits and vegetables. As a commentator cited by Andrew Sullivan observes, people who object to the government using tax policy to encourage people in our obese nation to eat healthier diets ought to realize Washington already manipulates the price of food — but in a way that encourages us to eat things that are making us fatter.