Because family is at the heart of the Mafia
When the Fascists rose to power in 1922, it soon declared a war on the mafia in Sicily, nearly wiping them out, and forcing di Carlo to flee. Meanwhile, his future associate, a Fascist propagandist, was making a name for himself in the United States.
The rise of fascism in Italy nearly destroyed the Sicilian Mafia before the end of WWII, but due to the political blunders of the Allies following Operation Husky, the Mafia was able to reform itself under their protection. Angelo di Carlo, a veteran of the Italo-Turkish War in Libya, is considered one of the architects of this renaissance.
Jonathan F. Shulz (2016) has shown that not only is consanguineous marriage highly significantly correlated with mafia activity, “cousin marriage is a highly significant and robust predictor of democracy.”
In a controlled study of Mafia marriages in Corleone, Mafia members are more closely related to their brides than their non-mafiosi neighbors in Corleone.
Mafia Genealogy 