The overriding objective of the justice system is to ensure that the innocent go free, not that the guilty are punished, and the system should be orientated around that objective.  Ex post-facto confessions do not make someone ‘clearly guilty’ as false confessions can arise for a number of reasons, from boasting to an innocent misstatement.  It is also wrong to assume that new evidence is better evidence. The longer a trial takes place after a crime the less strong the evidence gets; memories get weaker, people go missing, evidence can be damaged etc. There is also the problem that in a re-trial any tactical advantage of ‘ambushing’ a witness in cross-examination is lost because they know that the ambush is coming. There are therefore a multitude of reasons why retrials are less likely to achieve convictions than a well prepared first trial.