Prohibition was a failure because it did not stop people from drinking. Instead, people kept drinking and, in the process, enriched organized crime gangs and eroded respect for the law.
During Prohibition, many people kept drinking because they did not truly believe it was wrong and because the law did not make drinking alcohol illegal. But selling it was illegal and so criminal gangs got into the business of supplying alcohol to the people who wanted to drink. There was not enough law enforcement to have any chance at stopping this illegal alcohol trade.
So Prohibition failed because people kept drinking and because organized crime was willing to supply the alcohol. This reduced respect for the law and support for the idea of Prohibition.