There are several reasons, among others, are the following:
1- The main actions taken at regional and international levels are not for the elimination of the demand of drugs in the developed world but to reduce the offer in the producer countries. This is a wrong approach.
2- There are not sufficient resources allocated to help the poor farmers in the drug producer countries to change to other crops that provide them with the necessary resources to satisfy their family’s needs.
3- The drug business moves a lot of resources some of them used for the payment of the corrupted officials.
4- High level of corruption that exist in the drug producers and recipient countries.
5- Lack of collective actions taken to eliminate drug trafficking at all levels.
Better customs controls, better customs risk analysis, better fight for the growers and agricultural fields in the gtowing countries, eg in Peru, Colombia, Golden Triangle, Afghanistan....
Drugs are menace to the society. Drug trafficking is an international problem. Evil money earned from this is increasingly being used for terrorist activities across the world. To prevent this menace, the following steps are essential: 1. To create social awareness. 2. Strict vigil on vulnerable youths in various vulnerable places. 3. Destruction of drug producing agricultural fields. 4. Strict control by customs and narcotic control departments. 5. Exemplary punishment to offenders. 6. Providing help to poor and vulnerable sections of the society. 6. Rehabilitation centres must ensure that victims are properly reinstated in society after cure. 7. Determined and committed effort to eradicate this menace.
There is a lot of money in the drug trade by organized crime organizations.
Many states cannot ensure that agrucultural land is used for drug cultivation because they either do not care or have no control over their country or are under thread of armed forces under control of the organized crime organizations...
The money of the drug cartels is chaperoning their areas and intersts and growing grounds must be fought by money and customs forces and armed forces under control of the relevant states in question.
Possibly a common mission under UN control should be started...
In my country,We really have been successful on the war on drug during the 30 years from1950 to 1980. During the 30 years, we run Central planned economy in the whole country, so drug trafficking, drug planting.drug trading cann't exist in the country. After 1980 , following Free economy drug also come freely.
'War on drugs' should be led by taking the money out of it. To liberalize marijuana, allow that everybody can grow it then nobody won't be able to earn a single penny from it. I am aware that marijuana is not the same as heroine, speed, cocaine, etc., but surely it would make a difference, if the money would be out of these drug markets. Still the question and its solution is not so straight forward that it appears from the first part of my answer, but I trust that others in RG are able to contribute......