Because the situation with street drugs is completely artifically created by agressively policing them. If people want to take drugs they will take them and even the death penalty will not stop them. You might as well tax it, educate the children about it, be open about and soon we will find that it slips into a nothing. There will always be people who absue things, more people abuse alcohol than drugs. Making Alcohol illegal gave birth to the mafia, gave them huge power, and was a total disaster. The war on drugs is a joke, there is no war. Let people get high if they want to, its really not such a terrible social disease that people think it is.
It costs less than the price of a loaf bread for a heroin junkie to get his fixes for the day. Heroin is cheap as dirt. Obtaining it under such a harsh legal system is not, so a junkie is forced into a life of crime. Doctors that become addicted to medical grade opiates, often function perfectly normally for years before they are outed. Its not as bad for you as tobacco or alcohol, provided you have a supply, and once an addict is accustomed to the drug he can be perfectly functional in society.
Heroin is only expensive because there are armies of police, customs and law enforcement running about pushing up the prices.
The drug dealers do their best to bring the prices back down again, but of course charge a premium for taking the terrible risks they do.
The money they make goes into an uncontrolled violent world, that pays almost zero taxes and contributes almost nothing to society. Its a totally false economy that is stripping billions in cash each year from the real economy.