The fact is, the majority of the population favors keeping drugs illegal. If you want to change the law, all you have to do is convince people that drugs should be legalized. Few politicians are willing to bring up the topic of legalization because they know they will be voted out of office if they do.
That would indeed be the correct course of action, were it not for the fact that the government has a rather spotted history of providing the public with good research on the dangers of drugs. The perception of the public that drugs are dangerous is based, for a large part, on government funded research, and the government seems to be rather keen on finding something wrong with the drugs they're actively trying to ban.
As public policy expert Mark Kleiman once said: "There is a very, very heavy cost to the process of devaluing information that comes from the government. Lincoln was right. Government trust is a precious resource and there's a question about how much of it we want to squander in telling kids not to use drugs."