Folks on low income often do not have the luxury of being very selective in what they can eat. Their budget means they can usually buy once a pay cycle so food needs to be long lasting. Unfortunately a lot of the healthier options have short shelf life such as fresh veges and fruit. They buy what will last and what they can afford.
I don't believe that. It's about being educated about buying and preparing food and it does take some time. Fast food is and should be a luxury. In-season fresh and healthy food can be inexpensive but you have to know what you are doing. Countdown run a "Feed 4 for $15" promotion that is reasonably healthy (but does assume you have salt&pepper, milk, butter etc so is slightly underpriced). You can't eat dinner at Maccas for that little.
So many people these days have no clue as they were never shown by their own parents and listen to too many people who are saying it's excusable to live on junk because it's too hard and expensive to do otherwise.