I'm not sure I agree with this. Feel free to post some, I'm certainly willing to have my mind changed, but 1I've never seen any sort of compelling evidence that there is any correlation between economic status/level of education and substance abuse. I have, on the other hand, seen studies over the years that suggest that the middle/upper class consume drugs/alcohol at either the same rate or a higher rate than those at a poverty level.
On the other hand, there is a correlation with homelessness and substance addiction, but that adds multiple additional layers of hardship over simply being poor that would possibly lead someone to substance abuse.
I would guess the answer is much more simple, and that's just the bleakness of their existence. I live in a rural area of central TX, out in the country but near a town that is objectively dying and lacking opportunity for basically anyone that isn't a fast food restaurant owner or has some sort of specialized practice (dentist, lawyer, etc). I know so many people who are barely hanging on by the skin of their teeth and any number of major (or even minor, but seemingly major in their eyes) life events lie in wait that could absolutely push them into a hopeless spiral that could quickly lead to suicide.