Well one of the reasons is that they no longer have the ability to run things inside prison as in former days.( for example the scene in Goodfellas) Inside prison today the control is in the hands of ethnic gangs, such as Black and Hispanic gangs, so in order to survive the Mafioso would have to deal with one of these groups. We've all heard about how John Gotti had to pay the white prison gang the Aryan Brotherhood for protection. This is something a Mafioso would want to avoid. I was told that in most fed joints Mafioso look to Black gangs for protection since they feel they can control Blacks with money. So the long and short of it is that it's fear