Tommy Volker was in The Mentalist to show how untouchable a powerful man is , That is what i believe but at the end Volker had no one to help him get the dirty work done so that is what got him caught Red John on the other hand is a master manipulator he will always have someone to do the dirty work to make sure he is clean so even if Jane finds out who his is there will be no way to prove it to a court. 



  My theory  - Jane finds out who Red John is Lisbon tells Jane if he kills Red John she will lose him forever and she doesn't want to lose him the is no to convict Red John so the only way for Jane to stop him is to kill him at the Jane will have to choose Lisbon or Red John