Follow the money.

If you look into the history, you see that just before marijuana was declared illegal, there was a great push by many people to convert the production of paper from tree pulp to hemp pulp. It was cheaper, required fewer steps, and had less harsh chemicals involved. The same area of hemp produced many times the paper of that area of trees. Oh, and the paper was better quality. The campaign to make marijuana illegal was spearheaded by the son-in-law of one of the biggest lumber barons. Since you can't tell the difference between hemp and marijuana by looking at it, that meant hemp had to go.

They pushed the idea of it being dangerous even though the head of the AMA later said there was no problem with it. The tests they'd been shown to "prove" the danger and which they based their initial rulings on were flawed. Monkeys got brain damage and died from breathing marijuana smoke because that's ALL they breathed. They got no oxygen. That's not to say it's completely harmless. But the general view of modern pharmacology is that "If a drug is stated to have no side effects, then it is strongly suspected it has no central benefit."