As one of those twenty-somethings that grew up in the 90s, frequently used pain killers, and hated hard drugs, I’d like to suggest a more psychosocial-cultural hypothesis:
I am technically in the group labeled “Millennial” (I totally despise the term) and I, among others my age, feel a lack of control in quite a few areas of my life. Uncertainty about my future career, uncertainty about my financial status, uncertainty about what contributions I want to be able to make to my society. I know I’m not the only one who has thought, “why would I relinquish even more of my control by doing hard drugs or smoking like that?” What if as a generation we feel like we need to overcompensate for our lack of control by mostly eliminating our use of hard drugs?
As for sobriety, alcohol is nice but the effects last an unpredictable amount of time depending on what you drink, and no one likes a messy alcoholic. So pill-popping is a little more romanticized and manageable due to a fixed dosage.
I think there could be a combination of all of those factors that has caused Millennials to stick with the pain-killers and not so much any other kind of drug.