Scorsese is a good example that many great filmmakers follow..."Do what you know!"

Since "Marty" is a Queens-native, Italian-American descent, combined with his rabid obsession with movies, its not surprising he simply follows what he knows with stuff like MEAN STREETS and WHO'S THAT KNOCKING AT MY DOOR? and later on in RAGING BULL and of course GOODFELLAS and CASINO.

Its like John Singleton, an African-American filmmaker that made BOYZ IN THE HOOD and BABY BOY, two black "gangster" films, and then seeing some of Spike Lee's films(like in MALCOLM X and especially DO THE RIGHT THING) some of the protagonist's opposing foes were white. Because of this, are they anti-white? Of course not.

Besides, imagined if some California sunshine 20sh guy had directed GOODFELLAS. The end result is that the movie would not have been as "realistically gritty" and honest, in a sub-consious leve at least.

Same with Oliver Stone doing all those Vietnam War-set films....he was a volunteer in the Vietnam War, so hes doing what he knows.