Because the US Federal Government doesn't subsidize healthy foods ...
Physicians Committee for Responsible Medicine, Autumn 2007
... thanks to a political phenomenon known as Pork. Because a small number of farmers care greatly about farming subsidies while the rest of the American population care very little, and because a small number of politicians care greatly about being reelected, it turns out that the distribution of subsidies to farmers cannot be permitted to be influenced by vulgar considerations of healthy diet. You don't win votes with salad (see Lisa the Vegetarian).
via Consumerist, Farnam Street and the New York Times.
See also Pork Scratchings