I think a lot of people feel trapped in a life of crime by their past, and in the case of the mafia, by most of their friends and family. If they can get a new past and new surroundings, they may take advantage of the chance to live differently.
This is a version of the same argument that people make about rehabilitation generally: people with a felony conviction find it difficult to get good honest work, and many revert to living and hanging out with the same kind of people who got them in trouble in the first place. Witness protection is an extreme version of what most rehabilitation advocates are suggesting: an opportunity to start with a clean slate (i.e. no felony record that hurts employment), and avoiding bad influences from one's past. The results of witness protection seem to validate these arguments.
This is a version of the same argument that people make about rehabilitation generally: people with a felony conviction find it difficult to get good honest work, and many revert to living and hanging out with the same kind of people who got them in trouble in the first place. Witness protection is an extreme version of what most rehabilitation advocates are suggesting: an opportunity to start with a clean slate (i.e. no felony record that hurts employment), and avoiding bad influences from one's past. The results of witness protection seem to validate these arguments.