The factors led to cannabis being regulated differently and prohibited in the early decades of the 20th century were fueled by politics, greed and racism. Not science.
Cannabis has been used as a food, a fiber, and a medicine for thousands of years in many different countries and cultures. In the US it has only been prohibited last 82 years, starting with the Marijuana Tax Act of 1937. Prior to that, cannabis was part of the US Pharmacopeia and had been frequently used for analgesia, anti-inflammatory, and anti-spasmodic indications. In fact, US pharmaceutical companies such as Eli Lilly and Bristol-Meyers Squibb created commercial tinctures of Cannabis.
Harry Anslinger was the appointed commissioner of the newly founded Federal Bureau of Narcotics, where he drafted the Marihuana Tax Act. This law levied taxes on hemp and cannabis products, for which neither physicians nor pharmacists were exempt. Additionally, he used the Mexican name for Cannabis, Marihuana, an unfamiliar term to the medical community.
The AMA opposed this legislation. In a statement to the Committee of Ways and Means in the House of Representatives on May 4th, 1937, legal counsel for the AMA Dr. William C. Woodward, said, “There is nothing in the medicinal use of Cannabis that has any relation to Cannabis addiction. I use the word Cannabis in preference to the word Marihuana because Cannabis is the correct term for describing the plant and its products. The term Marihuana is a mongrel word that crept into this country over the Mexican border and has no general meaning, except it related to the use of Cannabis preparations for smoking. It is not recognized in medicine.” Woodward goes on, “To say, however, as has been proposed here, that the use of the drug should be prevented by a prohibitive tax, loses sight of the fact that future investigation may show that there are substantial medicinal uses for Cannabis.” 
Titans of the industry, such as William Randolph Hearst, supported making hemp illegal because it was a cheaper competitor to timber. He also used his newspapers as a platform to spread racist propaganda about Mexicans and African Americans, describing them as lazy and violent from smoking marijuana.
And just as, Dr. Woodward had feared, the final blow given to Cannabis as medication was done in the 1970s during Nixon’s presidency with the Controlled Substance Act listing Cannabis as Schedule 1, which is a “drug having a high potential for abuse, no accredited medical use and lack of accepted safety” thereby thwarting meaningful future research.
 Pasanti S, Bifulco B. Modern History of Medical Cannabis:From Widespread Use to Prohibitionism and Back. Trends in Pharmacologic Sciences. March 2017; 38(3):195-198
 Woodward W. Testimony House of Representatives Ways and Means Committee. May 4, 1937, http://www.druglibrary.org/Schaffer/hemp/taxact/woodward.htm .
 Frye P. Medical Marijuana Guide: Cannabis and Your Health. Lanham MD: Rowman & Littlefield; 2018 :6-8