But isn't the "poor worker drinking to escape reality" trope a trope for a reason? Plenty of people who are economically fucked drink and do drugs to escape reality - often budgeting for it as carefully as rent.
My first guess would be that being poor relative to peers is more stressful and depressing and relates to more substance abuse problems. When hardships are shared among whole generation it could affect them differently.
No idea, seems measuring these trends is easier than finding the underlying causation. Perhaps enough young people have seen the state of the local heroin addict and thought, ok so that's not really a good option either.