Because junk food is cheaper. $1.75 pizza slices will fill you more than a $1.75 worth of Broccoli where I live. Fast food places are also closer to them than grocery stores because they are out in the suburban sprawl so they go to what is accessible, and often those places run promotions for 2 for 1 burgers or whatever. I guess the difference is during the Depression there weren't any food subsidies so everything was expensive.