Because cannabis isnt a substance that we protecting from.
To legalise cannabis & tax it to create more money for the war on `hard drugs´. The money saved prosecuting, jailing & investigating cannabis users could be better spent elsewhere.
The prisons would be freed up of unnecessary inmates who would not fall foul of other criminals who may introduce them to other forms of criminality.
Legalisitation would mean the end of gangs & mafioso´s control & supply of cannabis.
People would have a choice between getting drunk & causing social & financial problems or having a civilised time with no hangovers or serious drawbacks.
You can buy paracetamol & kill yourself or others relatively easily, but with cannabis this is impossible, no person has ever overdosed on cannabis.