The 18th Amendment was repealed because too many Americans did not support Prohibition.
There were at least two reasons for this. First, there was the fact that not enough Americans thought that drinking was really such a bad thing. They thought that it was fun to go out to speakeasies and have some drinks and listen to bands play. When the majority of people don't respect a law, it can't really survive.
Second, there was the fact that Prohibition was helping organized crime. Gangsters were getting rich bootlegging. This led to a lot of violence that helped turn people off of Prohibition.
