There are at least several root causes. But the elephant in the room is that people who see no better way to feel okay about themselves WILL take addictive drugs if they’re available. And, once addicted, they’ll seek them out if they’re not available. Trying to make them less available has almost no effect except on the price. Let me say that again: Interdicting drugs has almost no effect on usage.
Twelve step programs work, when they do work, by waiting until the victim is near death and only when they’re psychically broken does the system of replacing the addiction with a pseudo-religion work. By changing how the person sees themselves and giving then a different (perhaps more benign) addiction.
Making housing, education and jobs available though: That works starting almost immediately. Because when you’re feeling like you’ve made progress and you can see a better future, you no longer WANT the drugs. Though you still have to deal with an addiction if you have one.
First step is giving up on criminal law as a solution. Second step is viewing it as a public health problem.
I was addicted to cigarettes and smoked for years. I then became addicted to some drugs and some alcohol. I quit the drugs cold turkey because I didn’t like the way I felt. I quit drinking for the same reason. In 2005 after over 40 years of smoking several packs a day-I decided to quit smoking and did it cold turkey. What I discovered for myself is I wanted to quit, so I did. Nobody talked me into it, I did because I WANTED TO.
All too often, people find it ideologically preferable to concentrate on the problem rather than the uncomfortable causes. This is especially true of conservatives, cf. guns.