Marijuana had no representation. The FDA was just set up and consisted of three people: one dude who "knew it all" and two guys who were learning from the dude who "knew it all". He had no knowledge concerning marijuana and when it came to classifying it as a safe or unsafe drug, the man chose to classify it as unsafe without any research to say why. This is what I've heard through the grapevine.
The racial issue was also another factor concerning marijuana and the mexicans, cocaine and the blacks, heroin and the chinese, and so on. The people in power found that the drugs were hindering the work they were supposed to be doing and so those in power took it all out of the picture by making them illegal.