1. No matter what the proof is, they will not believe it.
2. Even if he is guilty (as I believe he is) he is still sympathetic to their cause(es) and thus they will remain in silent denial.
Consider this also:
By going after Bill, again and again, we make Hillary out to be the victim, thus gaining sympathy votes.
Bill is history and his star diminishes further each time he speaks. Hillary is the future star of the Dems and she will drop him, like a bad habit, the moment she no longer benefits from the relationship.