Because the current tactics make criminals out of addicts and floods the prisons with unnecesary occupants.
The problem with many societies is that they sometimes fail to learn from the past. This is what is happening with the war on drugs. It is a decades long campaign that can never be won, at least not through any traditional method.
The law is what makes criminals. This isn't just a musing, it is literal. Because the law is shifting regularly, an innocent person can be MADE guilty. Prohibition of alcohol is what created gangsters like Al Capone and yet government seems to see no causality between prohibition and the insurgency pf people who find ways to capitalise on this prohibition. Law does not stop people acquiring and selling drugs, it just makes it harder, and makes its economic infrastructure more underground and thus more violent.
Some comodities are always in demand, drugs are an example of this and the mistake the governent makes is that they think its a real war, like the drugs and the dealers can all be killed, when these complex networks of drugs traders are more deeply buried into society than any terrorist network. Drugs trafficking bears more similarities to a business than a politically driven or idealogically driven terrorist cell.
Drug traffickers are a twisted hybrid of terrorist and business man, where terrorism and violence is company regulation. The only way for the government to fight this infectious threat is to redefine its war and this war is simple economy people! The governent undercuts the rival (the black market). You legitamize it, create recreational instutions where people can take drugs in an environment that is safe from the violence and criminaity of the underground it was once banished to.
Addicts need to be treated as "P.O.W"s rather than soldiers, customers rather than company workers. When you undercut the business man, and offer better services than him, he loses business.
Possible caveats: If it is legal what stops everyone from taking drugs?
Well, cigarettes and alcohol are legal but not everybody uses them. Sometimes people just don't like their experience on drugs and don't do them again. We should be looking at educating people on using them but not allowing it to consume them, unfortunately, in the "shadows of society" (to borrow a Cleggism), they go without guidance and drugs become all they know, resorting to crime to get it.