Demographics - old people, religious people, people whose jobs depend on prohibition and politicians that pander to old or religious people.
Ideas - The idea that alcohol isn’t a drug, the idea that drugs are a moral weakness, that drugs are cartoon evil waiting to steal children off the streets, that all recreational drugs are equitable in effect and addictive potential, that moderation is not possible, that drug use is something for which you must immediately apologize if you are in any kind of public role and that you must admit to be a ‘problem’ or else you can’t be trusted in that role (But you should definitely grab a beer in front of a camera to show how insert country here you are! Voters love that!), the idea that legal drugs would destroy the economy by creating a never ending orgy of hedonism, that while on drugs a person does not act in any way according to their sober self. The list can go on.
Institutions - Many politicians and their parties as a whole, many law enforcement agencies, Correctional and legal staff, Criminal organisations, Religious groups. All of these are majority against ending this so called war because it either benefits them directly not to do so or it otherwise somehow offends whatever version of magical thinking they identify with.