Ok. I've heard numerous theories about why it is illegal, but I'm pretty sure the real reason behind it's being illegal is the downfall of the economy which it would probably produce should it be legalized. The economy relies on a large part on pharmaceuticals being prescribed to medical patients. I'm almost positive that I read somewhere that weed was prescribed as a 'wonder drug' prior to it's prohibition. Since marijuana has so little side-effects (especially in relation to many phamaceuticals out there), legalizing it would make those medications far less popular than they currently are.
Since we rely so heavily on sellin' these, if weed was legal, most would probably switch to marijuana. And yes, many many people WOULD grow it, and you can't grow pharmaceuticals can you? Less money going into the system. Let's also not forget that we've tried to legalize it, and it didn't pass.. it's not only the government who doesn't want you smoking, it's your neighbor. Your teacher. Your parents. The people open-minded enough to realize the great and harmless potential of marijuana are few and far between. I'd say we're a minority.