Why is Marijuana illegal?

Seems like the vote in Colorado and here stirred up the hornet's nest and gotten the interest of the media.  Again, here's an article that claims the government's decision to classify marijuana as a dangerous drug was made arbitrarily with no scientific basis. 

I had heard this way back in my law enforcement career and wondered why it wasn't more of an issue.  I'm still wondering why there's so little said about why it keeps being driven into an illegal status which benefits the opportunists controlling the illicit market when it is so cheap to provide?

my best explanation is that extremely lobbying from tobacco companies mixed with an easy 'sell' on the idea that the drug is dangerous.
the idea of 'DANGER' is easy to sell because it is better to error on the side of safe. If you can argue something might be dangerous and can't be proven it is absolutely not dangerous then the damn DEA decided that they can increase their budget if it remains illegal. they would probably have to let go half of their staff if it was decriminalized and legalized. 

Is marijuana bad for you? - The Week