Nihilism and a lack of a reason to care about getting better. Drugs offer a cheap escape from their grim view of reality. No friends who aren't addicts, so it's normalized.
A criminal record blocking gainful employment. Court mandated treatment programs that prevent people from moving away from their deadend town and self reinforcing social circle of addicts. Court mandated treatment that's forcing them to spend all of their time around other addicts, who only have drugs in common and thus have their relationships built on the shared interest in drugs, preventing them from participating in anything that would lead to a more productive social circle.
Their criminal record might mean that by an enormous margin the most lucrative thing they can do, and perhaps actually their only realistic hope of financial independence, freedom, and an escape from their miserable reality of barely scraping by spending most of their time doing things they hate, is to sell drugs, which is self reinforcing their environment.