Historically, drug prohibition was often seen as an issue of morality, not a medical problem that needs treatment. Certainly when drug prohibition began (eg the early laws outlawing cocaine, opium, and alcohol), the driving force was almost always either religious groups reacting to social ills, or moral panics of some sort.
Recreational substances have been a favorite moral panic for decades. Even in the 1980s (crack cocaine moral panic) and to a lesser degree the 1990s over MDMA. And this goes back a long way: think reefer madness in the 1930s, the racially tinged moral panic surrounding the Harrison Narcotics Tax Act, etc.
I know some people in the comments below are pinning more blame on the bad laws on lobbyists and entrenched interests, that's certainly partially the case (it's certainly open for debate how much they contributed to creating the moral panic even). But I would tend to put more blame on the moral drive behind many of the laws themselves. Moral panics have a nasty tendency to create harsh legislation that often creates more problems than they solve. And while there are entrenched interests defending the War on Drugs, at one point they were popular entrenched interests. No one is going to overturn legislation when 79% oppose even marijuana legalization, as is the case for this poll in the US in 1988:
http://www.people-press.org/question-search/?qid=195853&pid=...
To get the "political will", I feel that you have to counter the moral panic that has led to these bad laws (as well as the perception in the public mind) with "just the facts" on these compounds (most aren't as bad as the demonization would have it)... combined with demonstrations of the harsh consequences of these laws.
That's already happened to some extent... in 2016, a majority support marijuana legalization. There are more articles then ever on the high cost of our incarceration policy, and there's even greater awareness of how our drug policy has contributed to international violence.
I'm not sure that all Schedule Is will ever be legalized fully (personally I'm on the fence with many of them on to what degree they should, since some might indeed pose more of a risk to public health than it's worth). But if only the incarceration policy was adjusted to more of a treatment based one, that would do a heck of a lot.