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.

I'd say the consequences for violating the witness protection program contract play a huge role in enforcing its respect.

Nitpick, but "by definition" career criminals is incorrect. They are by definition witnesses, 5% or so of whom have done nothing wrong.

If you commit a crime on witness protection you would be cut loose. Then the mob can come and tidy up. Being killed by a mobster is probably pretty good motivation to not commit crimes.

Reminded me of this article (link below with TL;DR). So we have two strong examples of how changing environment can reduce crime and improve lives. Perhaps we should change our current educational and judicial policies?

TL;DR: after hurricaine Katrina, many people who relocated from the lowest-income, highest-crime areas in New Orleans (and many of whom had previous convictions) improved their circumstances--reduced recidivism, higher standards of living, etc. http://www.newyorker.com/magazine/2015/08/24/starting-over-d...