I thought of some reasons why marijuana is illegal...
1. An excuse to keep a powerful Police State, give more power to the government and thus control over the masses in all other areas of life.
2. To keep alternative medicines out of the hands of the masses and to keep Big Pharma more profitable.
3. Marijuana mellows the masses from working too hard, makes them more peaceful, and gives them ideas about how they are being oppressed (alcohol and cigarettes make the masses work harder, create more conflict, and cope with being oppressed).
4. An excuse to keep Hemp from being used in place of the patented profit making textiles and petroleum based products like plastics created by Big Oil.
5. To keep the "peace movement" from gaining ground. War and conflict is much more profitable.