Because the only things the criminalisation of cannabis has done is increased the amount of criminals on the street, made these criminals richer, caused growers/dealers to contaminate their cannabis with glass, sugar etc, prevented the police from protecting people from real crimes, allowed cannabis to become a political scapegoat, and allows young people to smoke the plant. Amsterdam had the right idea and look how its worked for them. They have one of the lowest crime rates and lowest amount of hard drug users.
In todays society is it really fair to legalise a plant that nearly every single person in the country has used? Im seventeen years old and tried my first spliff at 13. Every single person i know smokes it and its even easier to get than alcohol. Why? BECAUSE its illegal. Alcohol is so hard to get when young because a person who serves someone who is underage, can get a criminal sentence but they can still make a profit from selling alcohol to people legally allowed to consume it, therefore they are more likely to enforce the law themselves. Thats why the alcohol laws work. sort of.
Cannabis is by far one of the most softest drugs on the streets. Your childs baby food probably has more harmful chemicals in than cannabis. It can also be used for medical purposes and hemp can be used to make biofuel, fabrics, robe and is an eco friendly alternative to paper. If cannabis was used in the same way as alcohol, not only would less people be using it but it would make it safer, reduce crime, prevent young people from using it and, the huge amounts of money that cannabis makes, wouldn't be going into the hands of the criminals, but would be put back into society in the form of taxes etc.
List of drugs that have caused more deaths than Cannabis (in no particular order):
Heroin | Caffeine | Aspirin | nicotine | Cocaine | Alcohol | Methadrone (legal highs) | ecstacy