There were a number of reasons why Prohibition was abolished. The basic cause of its abolition was that people were no longer convinced that Prohibition was doing more good than harm.
One problem with Prohibition, perhaps the most important, was the fact that it was widely disobeyed. As it turned out, too many Americans did not think that it was wrong for them to drink. They might have thought that other people drank too much and in the wrong circumstances, but they did not feel that they should abstain themselves. This led to a situation in which huge numbers of people were simply disobeying the law, not even feeling that they were doing wrong. In such cases, a law cannot last long.
Another problem with Prohibition was that it ended up helping organized crime. Since there were still so many people wanting to drink, there was a great deal of demand for alcohol. Since alcohol was illegal, it was organized crime that had to provide the alcohol to meet that demand. The law, then, was helping to enrich violent criminals.
Finally, Prohibition was costing the taxpayers money instead of allowing the government to make money in taxes. Legalized alcohol leads to companies paying taxes on the money they make selling alcohol. Prohibition ended this and it also incurred costs as the government tried to enforce the law. This meant the government was losing money enforcing a law that too many people did not even believe in.
These factors led people to feel that Prohibition had too many costs and not enough benefits. This led to its repeal.