William, marijuana was made illegal in 1937 for 3 reasons:
1. They didn't want to lay off the prohibition cops after prohibition's repeal so they turned them loose on marijuana.
2. William Randolph Hearst owned large forests that he chopped down to make paper for his newspapers. He didn't want the competition from cheaper, more environmentally friendly hemp based paper.
3. The DuPont chemical company wanted to sell nylon ropes to the navy who had previously used ropes made of hemp.
So, they made marijuana illegal. The government wasn't interested in helping people, only themselves.