The probable reason is that, in the US, kidnappings always bring in the FBI, and the Mob doesn't want to provoke the FBI if they can avoid it.
The Gallos kidnapped the Profaci Family hierarchy in the early Sixties, but they weren't exactly "civilians."
Same thing with Contract killings. The Mob didn't do that because it would expose them to civilians and it would attract the FBI.
Sammy Gravano's crew was approached by a lawyer who wanted them to kill his wife. Sammy didn't want nothing to do with that.
One of his soldier Huck Carbonaro wanted to tell the guy he would do it then keep the money and do nothing but Sammy refused. I don't think it's an ethical thing, it's just that you don't mix the Life with civilians and you don't kill people for money.
