I think a lot of it is convenience-related.
Why go home and set forth on a good hour or so's worth of food prep/cooking when the chip shop is just right over there and woohoo, look, there's nobody standing in line, better run quick!! Regular potatoes [when had often and alongside other starchy carbs] can do a lot of damage, long-term, to ones health. In fact, ALL the white stuff: flour, sugar, rice, taters, it's all a glycemic index nightmare!! And that's what a lot of people, regardless of economic status, [but probably a little more prevalent among the poor? Certainly so among the poorer informed] end up having as the foundation of their diets.
Some of that could be influenced by pricing. I have seen at several different times in the past that some healthier alternatives such as wholegrain/wholemeal breads, brown rice, honey [especially raw and/or organic], sweet potatoes, etc. can be priced higher than their white, refined and more common varieties. Not always, but noticeably so. I think that if a person wants to eat well and they do a little research and shop smart then they can get by better off than those that just go into it all willy-nilly and sometimes even at a bargain. Not to say that it can be done everytime or long-term [grocery shopping has nearly doubled in price over the last couple years!!] but you
can do better
and get by here & there when you plan better and just make smarter/healthier eating choices/purchases.
But it does seem nearly impossible to come out on top [money-wise] when trying to be more health-conscious. Even when making changes from white to wheat there's still the factor of E-numbers, yeah, fucking E-numbers in fucking bread?!?!?!?! Not to mention all the other 10-letter ingredients in just about everydamnthing in the store: hydrogenated this, maltodextrin that, rapeseed oil, yeast extract powder [hidden form of MSG/free glutamates], high-fructose corn syrup, that old ambiguous bullshitter of a word "flavorings" that doesn't sit well with me and the list goes on and on.
Additives, preservatives, sweeteners, enhancers, chemical poisoning everywhere you turn!!! I've cut out sooooo much stuff that I really love and still there's a laundry list of shit I'd be better off not eating that I still do. But I can't just go completely "clean" with my diet. It just wouldn't work. As much as I hate it, I'm bound to it to a certain degree. At least until I can readily let go of those things. I can't just yet. Life's not worth living if ya can't look forward to a bacon cheeseburger once in a while!!!