I remember reading an article a while back stating that the modern Mafia really won't kill someone unless they really need to. As the OP stated, forensic evidence and better prosecution methods make murders more risky than ever before.

I also suspect that general cultural assimilation may also play a part in the decline of violence. Remember that most Italian immigration occurred in the U.S.A between 1870-1920. Many immigrants lived in impoverished neighborhoods where gangsters were viewed as the most successful and wealthiest. Although most Italians were legitimate, there was still an eager supply of recruits that were willing to break legs and kill for some easy money. Nowadays Italian Americans are far more educated and legitimately prosperous than say their grandparents. They don't need to resort to a life of crime to make a living. Plus traditional Italian American neighborhoods such as Besonhurst have been phased out with other groups (having an ethically uniform area could bred loyalty and a distrust of outsiders, which the Mafia capitalized on for many years) Add to that that sucessful law enforcement infiltration (Donnie Brasco and Jack Falcone) make mobsters less eager to seek out new recruits. Hence, why La Cosa Nostra has been contracting for the last 30 years or so.

The point is that guys probably figure that they can't resort to killing all the time, because there aren't enough young bucks to replace dead wiseguys than in the past. Even John Gotti over twenty years ago commented that new recruits were getting harder to find when he was in charge. I'm sure some young kids in New York still want to join up, but there just isn't enough of them to keep disposing older mobsters at a high rate.