Why is it that the streets of Naples in the south if Italy are covered in graffiti but those of Milan in the north are clean of this?

Same reason Manhatten is relatively clean and poorer boroughs aren't. Southern Italy was always agricultural and poor. Northern Italy had more industry and more wealth. People in poor areas suffer more from despair. And affluent areas are better able to clean up the graffiti. Most immigrants to the US after WWII came from southern Italy because conditions there were so dire. Do a little reading.