You've hit upon a reality of American life: the movie industry is white-dominated, most movie-goers are white, and therefore most films that expect to make big money stick with white casts and white themes. Another (unfortunate) part of movie culture is that scriptwriters and directors tend to go with stereotypes, rather than show nuances of character. So, if they need a racketeer, they go with an Italian character, a drug dealer is most often black or hispanic, a drunk is Irish, a landlord or storeowner is a Jew, etc. When I was a little kid, my mother (who was a grade-A movie fan) always told me that the only time you saw blacks in films was in prison movies. She was right.

As for Scorsese: as others have pointed out, he goes with what he knows. "Mean Streets" had a black character (a dancer that Charlie wanted to employ in a restaurant he hoped to take over). Scorsese collaborates with famous black director Spike Lee. You often see Scorsese's "stock company" in Lee's movies. Watch "Jungle Fever" and you'll see much of the cast of "Goodfellas," and not just Samuel L. Jackson either.