I think a large number are there precisely for that reason. Law enforcement, for instance, detectives and other investigators, will look for individuals who have been compromised within their own organization. These are the people they can turn. In fact they are often looking for people who have fallen from grace, who have lost some drugs or misplaced some money or whatever, and all of a sudden they need help. Clearly these are cases you will find in a witness protection program.
I would ask why you mostly find criminals in the witness protection program and why the innocent victims are not getting any protection. There are two potential reasons for this. One is that they don't need it as much, and second, sometimes a softer method of protection for a short period of time is sufficient. I buy those arguments, up to a point.
There are other people who really need protection and never get it. They don't get it mostly because of cost and because the logic of those police-based programs is that you don't invest in that witness if you don't need the witness. These programs are seen as an investment in a case, as opposed to extending protection to someone who comes forward either to cooperate or simply to ask for protection.