In addition to immigration, I want to add several other factors that made organized crime--Mafia and other ethnic gangs--take hold in America:

First, at the turn of the 19th into the 20th century in America, big business was forming ruthless cartels and monopolies that squashed competition by force and corruption. They provided a strong economic model for organized crime.

Second, Italian, Irish, Jewish and Chinese immigrants to America generally regarded the authorities in their home countries as enemies, making them easy prey for gangsters of their own kind in America. The gangsters knew that their own people would never complain to the police. That's how they flourished in their own neighborhoods.

Third, Prohibition provided gigantic opportunities for organized crime gangs to make money on an unprecedented scale. Since few Americans supported Prohibition, criminals had little trouble in corrupting police and politicians at even the highest levels to protect their booze rackets. The money and influence they made helped the gangsters to spread out into other fields.