Cannabis was placed on Schedule 1 under the Controlled Substances Act of 1970, but its use has been restricted since 1937. Schedule 1 prohibits use because, allegedly,
"Substances in this schedule have no currently accepted medical use in the United States, a lack of accepted safety for use under medical supervision, and a high potential for abuse.
“Some examples of substances listed in Schedule I are: heroin, lysergic acid diethylamide (LSD), marijuana (cannabis), peyote, methaqualone, and 3,4-methylenedioxymethamphetamine (“Ecstasy”).” (From MedShadow)
Note that these substances vary widely by effective dosages, physiological and psychological effects, and potential for addiction. In other words, it’s a decision based in politics rather than in medicine.