Edited To Add: I will add the caveat that my family and friends are middle class or above, and I agree with the suggestions above that the poorest folks tend make bad food choices at least partially because crap processed food is cheaper.
This is what I'm thinking of most, I guess. There's the prohibitive cost of better food (at least that's the way it is now, I don't know enough about food production to know if it has to be that way or not), and then there's also the mindset.
I don't know a non-dick way to say this, so I'm just gonna put it out there. We had a lot of poor patients, and their financial situation was not the only difference between them and the average middle class patient (or middle-class person I've ever known). They were resistant to quite a lot of things, because quite a lot of things didn't make sense to them, no matter how simply you explained it, if you get my drift, and also there's just a mindset of "I don't care about that, so don't bother me with it." They might be real interested in Real Housewives, but talk to them about their cholesterol (or politics, or books, etc.) and their eyes glaze over and they let you know real quick "we ain't troubling ourselves with that." If I had to guess, I'd say indeed, their financial status was not unrelated to their mentality/learned behaviors.
I'm generalizing, though, so there's that. We certainly had lower-class patients who weren't the brightest who were happy to hear what the doctors had to say and did their best to comply, if they could afford it. We also had a few upper and middle class patients who appeared to struggle to match the intelligence of a brick (some of them were on too many meds for too long, tho, think, the Lexapro/Xanax crowd). And of course, some lower-class patients who were quite intelligent. I'm just saying, on the whole, there was a whole different mentality and way of life there.