The Godfather set the standard, then Hollywood lore just adopted the theme. It became common for writers to just refer to Italian gangsters as "Sicilians". Scorsese mostly avoided it. Just a quick line where Henry Hill talks about his mother and then later about being made. The Sopranos went the opposite direction and made a point of Tony's Neapolitan background, which I agree was a refreshing take for movies or television.
It’s also ironic that the mastermind behind The Godfather was Neapolitan.
Mario Puzo of course.