Junk foods at high-availability, low cost, offering low-to-zero satiety per-calorie, and habit-forming addictive potential. The answer is pretty much in your sentence. Add to the fact, eating well on a budget requires you to prepare food at home. Lots of pressures funnel people to obesity. In the before-times (if you want to set your clock well before the obesity epidemic), two-income households weren't a thing for the most part and manufactured junk foods were a novelty on the market. People kept more active even at leisure, with tv being the novel and singular screen-based entertainment device that not everyone could afford at outset.