One reason is the more you're forced to work, the less energy, and, more importantly, time you have for preparing healthy meals. Therefore you're more likely to go with preprocessed, prepackaged meals or straight fast food which will make you fat fast.

I've got a relatively cushy job and cook my own, relatively healthy meals but even I find myself going for that store-bought pizza when I have a particularly busy week.

Moreover, unhealthy, preprocessed meals are basically drugs in terms of the dopamine hit, so you're more likely to go for them if your life sucks and you're sad about it.

Add to that the lack of education on healthy eating, which I'd bet is easier to come by when you're at least middle class and bam, you're a fat poor person.