There are many and varied reasons for this trend. One, of course, is that McDonald's has more control over the market than the individual consumer. But government policy comes into play as well; in particular the Farm Bill and the subsidies that support different types of food production in vastly different amounts. The Farm Bill provides billions of dollars in subsidies primarily to huge agribusinesses producing feed crops for animals, which, in turn is a subsidy for meat and dairy production. Fruit and vegetable farmers on the other hand receive less than 1 percent of total government subsidies from the Farm Bill.
This and much more information on this disparity is nicely presented in an article from Good Medicine magazine, a publication of the Physicians Committee for Responsible Medicine. Also from the article: