It became illegal because when it came to a vote pretty much no one in congress had even heard of it, they didn't know what it was. The few people who did know what it was also had something to gain by it being illegal.
Since most didn't know what it was or what it could do, they just took the word of those who wanted it illegal and voted with them.
Now that it's been proclaimed illegal and all the anti-marijuana propoganda has been brainwashed into people minds it's hard to convince them otherwise.
I think we're going about it all wrong. We need some reverse psychology on this. We need to present it as , we've discovered a "new" plant. It can be made into : bio-fuels, clothing, food, medicine, etc. List off all the things it can do, but never tell what it is. Keep people in the dark about it, hype it up for a while.
Then when people are at the height of their excitement, tell them that this plant has been known of all along, and it is, Cannabis..........