It's because of culture, and also the history of the ethnic group in general. The Italians formed secret societies in Italy before it was unified due to foreign occupation and exploitation. They eventually learned how to just survive using any means necessary and learned how to gain real power, economically and politically. They knew how to form allies and organize as well.

Most African American crime gangs are probably very loosely affiliated with each other and therefore therefore more violent and less powerful. The black gangs are quite isolated from society I think as well.