Alcohol was illegal, people wanted to drink booze with a passion, and the market arose to supply the desires of drinkers from all walks of life. Prohibition was a failure because the people of this nation dearly like to get buzzed on hooch.
Prostitution is illegal in many states.
Sex probably rates higher than whiskey in desirability, and so organized crime moved into illegal prostitution. Demand creates supply. Gambling is very popular, and is illegal in many states. Wherever you can't legally bet on a horse race or a ball game, you will find a bookie system that illegally provides the gaming customer an outlet for his or her needs. And finally, mind-altering drugs are an American way of life.