It's a complicated question, but the short answer is this: an unpleasant mixture of fear, misinformation, racism, stubbornness, inertia, and plain old stupidity.
We'd save tons of money on enforcement, prosecution, incarceration, etc. if we'd just legalize, regulate, and tax the stuff. Plus we could use the by-products as biofuel and textiles. But we won't, because we're scared, misinformed, and stupid.