Prohibition, the banning of the manufacture, distribution, sale, or consumption of alcoholic beverages, was doomed from the beginning for a variety of reasons.
Historically, alcoholic beverages had been present and important since colonial days. Directions for construction of stills and distilling of alcohol were easily available; stills could be located in any hidden part of a home or rural property. As production from some stills grew beyond the amount needed for home consumption, organized gangs began efforts to control and profit from the distribution and sales of the alcohol. Gangs used bribery and force to intimidate those charged with enforcing laws against alcoholic beverages, leading to violent deaths and great wealth for gang leaders. Gangs also became involved in importing alcoholic beverages from overseas.
As the Roaring Twenties went on, being able to trick the authorities and drink became a sign of personal liberation and freedom. The thrill of doing something that was technically illegal was seen as exciting and adventurous. Law enforcement authorities were overwhelmed by the challenge of trying to prohibit an activity that most of the public came to believe was only slightly naughty.
The reason the Twenties were Roaring was not just due to the post World War I economy in the United States, it was due to the increase in alcohol production. The reason Chicago and Al Capone became the iconic representation of the era was that Chicago became the major alcohol importer for the United States, getting "legal" liquor across the Great Lakes from Canada. The reason the United States still has organized crime is that organized crime got its start during Prohibition.
One cannot regulate human behavior, one can only mitigate its effects. The enforcement of Prohibition failed because most people in the US didn't want it, and all aspects of alcohol production and consumption went underground.
Prohibition was very difficult to enforce based on many different factors. First, the United States shares two extensive boarders with other countries that at the time did not have laws prohibiting alcohol. The US borders with Canada and Mexico have always been points of trade, and given the natural cross boarder connection combined with supply and demand; alcohol was easily brought over borders. In the Detroit, MI area for example there are several islands in the Detroit River and mouth of Lake St. Claire, these islands are easily reached from both the US and Canada by small boat, and Canadian Club Whiskey is and was made right across from one of these islands. A second and just as pertinent reason why prohibition was so hard to enforce is because different types of alcohol have been made from local ingredients in the home setting for the entirety of our national history. It would be nearly impossible to stop every single American who has the ways and means to produce homemade alcohol. Add to those factors the vastness of the country, the small percentage of law enforcement and the general lack of support for the law and that is how prohibition crumbles.