It had everything to do with the king of yellow journalism newspapers
not wanting competition for his yellow paper and the king of the new
plastics not wanting competition with them: competition from hemp.

Kathleen Murphy wrote for the Washington Free Press 3 June 2009 about

How Marijuana Became Illegal,

As the methods for processing hemp into paper and plastics were becoming


more readily available and affordable, business leaders including William
Randolph Hearst and DuPont stood to lose fortunes. They did everything in
their power to have it outlawed. Luckily for Hearst, he was the owner
of a chain of newspapers. DuPont’s chief financial backer Andrew
Mellon (also the Secretary of the Treasury during President Hoover)
was responsible for appointing Harry J. Anslinger, in 1931 as the head
of the Federal Bureau of Narcotics and Dangerous Drugs.

Anslinger and Hearst made up whatever propaganda they thought might
scare the public into supporting prohibiting hemp:

Hearst’s papers deplorably published enhanced accounts of marijuana-crazed
black men raping white women. With these sensationalist newspaper stories
as his support, Anslinger testified before Congress that, “Marijuana
is the most violence causing drug in the history of mankind.”

Anslinger completely contradicted himself later—before Congress
again in 1948—when he testified that marijuana caused its users
to become peaceful and pacifist, and that Communists would use marijuana
to weaken America’s will to fight.

Racism, communism, whatever was available for their propaganda: they used it.

Oh, yeah, and change the name to confuse the real experts:

A very interesting piece of that history is that only two days before
the 1937 marijuana hearings, the American Medical Association (AMA)
had just realized that the plant that Congress intended to outlaw
was known medically as cannabis, which from 1850 to 1937 had been
recorded as being the prime medicine for more than 100 different types
of illnesses or diseases in the US pharmacopoeia. Dr James Woodward,
who besides being a physician was also an attorney, testified that there
wasn’t any real evidence being used to justify the new law and that the
whole reason the AMA hadn’t come out against the law sooner was that
“marijuana,” the new name given to cannabis by Hearst papers,
was always described as a “killer weed from Mexico.”

Now cannabis and other drugs are being kept illegal to feed the
prison-industrial complex,
including CCA,
which admits in its 2010 report to the SEC that legalization
or even sentence reform would adversely affect its profits.

We don’t need a private prison in Lowndes County, Georgia.
Spend those tax dollars on rehabilitation and education instead.

-jsq