Maybe because "Sonny" is a legend and out of respect for him, Michael is relatively safe until the Sonny passes away. Just my thought, but then again, Gotti Jr. is left alone so maybe Neo is right. The mob they were a part of and the guys who were real gangsters, the ones they betrayed or left behind are long gone, dead or in jail if not then they probably just want to spend their last days free so they just don't care to take revenge. Good question tho.