People have wondered why Scorcese and Coppola keep coming back to mobsters, specifically the Mafia, and all that corruption mixed with loyalty. Of course no one knows, perhaps not even the directors.
But I suspect there are at least three reasons: First, it works economically. It draws the audience in. There is power and violence and sex to appeal to the men's fantasies. There is loyalty, intrigue, sheltered family lives, and the excitement of a wealthy rogue male for women.
Second, Italian-Americans may return repeatedly to stories of the Mafia for the same reason that Southern writers deal over and over with themes of minority relations. In spelling out the stories, they are explaining the odd customs of their subcultures to the rest of us. Yes, we are hung up on these obsessions, just as you believe, and here is how things work -- a bit more subtle than you'd thought, aren't they?
Finally, in exploring these themes in the interests of the outsider's better understanding of them, they can explore their own conflicts in model form, as stated in the anthropologist John M. Roberts' conflict-enculturation hypothesis. When we view something with ambivalence we tend to poke around in it, to touch it, fascinated by it. If we can't actually DO the thing itself -- become a mobster, endorse slavery -- we can play games with the idea, constructing little mock worlds in which we can jiggle the concepts around. In conflict about having children? Well -- you don't really need to. You can nurture something that is a model or surrogate child. Cats and dogs will do fine. But if your conflict is more intense, you are driven to less recognizable models. House plants, say. And if the conflict is VERY intense, you can start a coin or a stamp collection. What, after all, do you do with a stamp collection? You conceive of the idea, you start it when it's small, you feed it, you watch it grow and get bigger, and when it gets too big, or when you lost interest, you sell it on eBay and it moves out of the house. Directing a movie means constructing a model world too, like the model social world of a chess game.
In a movie, you lay out a plot, cause the characters to do certain forbidden things, and imagine how you might feel if you were doing them yourself. I'm not surprised that Scorcese made this movie, or that he's filled it with Italian-American actors. I'm not surprised that Faulkner wrote Intruder in the Dust, or that Judith Rossner wrote Looking for Mr. Goodbar, or that Philip Roth wrote Portnoy's Complaint. These model worlds they've created deal with issues like crime, race, female sexuality, and ethnic loyalties that have kept Italian-Americans and Southerners and feminists and Jews marginal to the rest of our culture. The artists are trying to explain to us and to themselves how things look from the inside. If these issues ever disappear, which is most improbable, the conflicts will ease, marginal populations will no longer be prompted to produce such intense work, and the rest of us will be poorer for it.
Maybe in this case Scorsese did too good a job of it. I found myself listening to DeNiro's voice-over telling us that the James Woods character was a "scumbag" because he was a small-time chiseler chasing dentists on the golf course, taking money from the whore Sharon Stone. But who is DeNiro's character to make such judgments? Woods is detestable because he's a vellicating minor hood with no money? Whereas DeNiro is morally superior because he's a major player with LOTS of illegal gains? We admire DeNiro because he is the more successful crook. The moral calculus is a little confusing.
On top of all that highbrow stuff -- man, does Scorsese know how to stage a fierce family fight. I should know. And that opening scene -- a car explosion backed by the majestic sweep of Bach's Missa Solemnis!