Another is time: minimum wage for a 40 hour work week earns you less than $15,000 a year (in the US). So many working poor are working more than 40 hour weeks, and can't take the time to cook, even if they can afford and obtain ingredients. Fast food is cheap, available, and fast.
Bigger problem is that low-income folks don't have the time, the access, or the urge -- marketing aimed at low-income folks works as well as on any demographic -- so they miss out.
If you're working 49 hours a week in 2 jobs and taking the bus home, are you gonna have the time to make steel cut oats and organic quinoa with mushroom broth, or are you going to hit the easiest thing on the way home, aka a quick burger at McD's? Quinoa ain't bad, but it doesn't hit the spot a the way a burger does either.
If we're talking the cheapest and worst food habits of the poorest, we're talking lots of ramen and tons of frozen foods. You can get big bags of frozen french fries for way cheaper than fast food. $3 gets you a fry at McDonalds or a 2 pound bag of frozen generic.
Fast food would be the luxury version lol.