Syncretism, and Italian regional differences. The vast majority of Italian Americans are Siciliano or Napolitano (except around here, actually, we got a lot of Genevesi and Lombard immigrants in Northern California). I have a friend from Bergamo that swears that nobody in Italy uses garlic or tomatoes in cooking and she gets actively angry about Italian-American food, bit other friends from other parts of Italy who say Bergamo is practically part of Germany...
But yeah, a lot of Italian immigrants had to improvise with what ingredients were available locally, hence stuff like cheddar "Macaroni and Cheese."