Actually Wikipedia has a very informative article on: Legal history of cannabis in the United States - WikipediaThe reasons it was made illegal in the first place are highly specious. I believe it was the late 30s or early 40s(I could be wrong) when Reefer Madness, a "documentary" about the horrors of marijuana (so named to make it more Mexican/dangerous sounding;it's actual name is Cannabis.) was released.
(It debunks the oft-repeated story on the Hearst/DuPont wood pulp angle.)