It is easy to start with drugs and then expand to other areas. Drugs are much much easier money than kidnapping or ransom. Once you hit the ceiling with the drugs it is only natural that you'll look for something else and you'll have money to fund it. Without the drugs it would have been much harder to organise other source of income. I think by delaying repeal of prohibition most of the countries risk to have their cartels grow to the size of too big to fall. I think in many countries this has already happened - especially those where any logical debate about legalisation of drugs cannot be conducted. The corrupt reasons behind keeping it up are just too obvious, but nobody dares to change the status quo.