* The rise of law enforcement. In the early 20th century, law enforcement agencies were relatively weak and underfunded. This allowed gangsters to operate with impunity. However, in the 1920s and 1930s, law enforcement agencies began to receive more funding and resources. This made it more difficult for gangsters to operate, and many were arrested or killed.
* The end of Prohibition. Prohibition, which banned the sale, manufacture, and transportation of alcohol, was a major source of income for gangsters. When Prohibition was repealed in 1933, this income stream disappeared, and many gangsters were forced to find other ways to make money.
* The Great Depression. The Great Depression, which began in 1929, had a devastating impact on the American economy. This made it difficult for gangsters to find customers for their illegal goods and services. Many gangsters were forced to close down their operations, and some even committed suicide.
* Public outrage. The public was increasingly outraged by the violence and corruption associated with the gangster era. In the 1920s and 1930s, there were a number of high-profile gangland killings, including the St. Valentine's Day Massacre of 1929. These killings shocked the public and led to calls for tougher law enforcement.
As a result of these factors, the gangster era came to an end in the 1930s. Although there were still some gangsters operating in the years that followed, they were no longer as powerful or influential as they had been in the 1920s.