As I understood it the movies about the Italian Gentile mafia were inspired by the Jewish gangs and or the Jewish Italian mafia.
But people were afraid to make it about Jews so they changed the characters to Italian Gentiles.
I also heard the Italian mafia has people perform rituals that are considered blasphemous to Italian Roman Catholic Gentiles
Jews have specific rituals to mock Jesus Christ
So the explanation that they are Jews not Catholics makes more sense