Prohibition of anything, whether it's alcohol, illegal drugs, the sex trade, or gambling, is one of those things that looks good - depending on one's outlook - on paper but impossible to accomplish. There are many reasons for this, some of them relating to economic realities, but most of them boil down to this: human nature.
1) There will always be people who do not believe in rules.
2) There will always be people who live by the motto: "If it feels good, do it."
3. There will always be people with compulsive traits, be it for gambling, sex, drugs, or alcohol.
4) You can't legislate morality.