After all, you don't even know why it was prohibited.
And how could you know that he doesn't know? Historians have written hundreds of books on the subject. It appears that your knowledge of 20th century American history is weak; you might start by reading up on this guy. He was the assistant prohibition commissioner in the Bureau of Prohibition, before being appointed as the first commissioner of the U.S. Treasury Department's Federal Bureau of Narcotics (FBN) on August 12, 1930.
He was the one behind marijuana prohibition. Books I read (granted, four decades ago) stated that there was an either real or perceived heroin problem, and since Congress wouldn't cough up more cash for his fight against heroin, so he made up the marijuana menace out of whole cloth so he could divert funds appropriated for marijuana prohibition to fighting heroin use.
At the time, about the only people who used marijuana during the '30s were blacks, Mexicans, musicians, writers, and artists, all of whom were irrelevant in politics at the time. Watch Reefer Madness, an anti "muggles" (slang for marijuana at the time) propaganda film. It's actually funny, especially if you're stoned.
One or two books I read had the Hearst family's fortune tied up in industries that competed with hemp: cotton, paper, plastics (rope), etc, but those books had no citations so were a bit suspect.
Now, here's a bit of history I lived through. I don't think I even heard of marijuana until the sixties when I was a teenager. What happened was white kids started coming back from Vietnam, where the black guys introduced the white guys to that killer Asian bud, and once home it quickly spread to other young whites.
And the anti-pot hysteria continues, despite any proof whatever that marijuana isn't the least dangerous of all psychoactive substances and even safer than many over the counter medications, such as aspirin or Tylenol (acetaminophen), the former of which can cause stomach and intestinal ulcers, the second of which can damage your liver.
The newspapers have been reporting on two people having psychotic episodes after eating huge amounts of pot, without noting that crazy people use pot, too, and one of the two had other psychoactive drugs in his system.
There was a fellow I was stationed with in the USAF who merely thought he'd been given LSD, and that was enough to trigger a psychotic episode. Crazy people are crazy, drugs or no drugs.