Please please PLEASE: this question is made from a place of pure curiosity, with no political biases intended. I’m asking because restrictions and prohibitions puts a pressure on the market since the demand never ceases, and someone interested in supply for that demand will do it for massive profits - no matter what. Wouldn’t it be the case for any product, be it drugs, alcohol or guns?
The other part of drugs being illegal is that addicted users may resort to crime in order to continue the habit. Decriminalization or legalization makes those drugs less expensive (ideally), which means less incentive to commit adjacent crimes.