The idea that it’s more Important that the process is followed and then stops at some point than that justice is achieved.
Same reason they barred introduction of new evidence when appealing from state court to federal, giving potentially corrupt state courts full power to block exculpatory evidence to deny someone justice because the federal courts must uphold the verdict if the evidence which was accepted indicates guilt under the state law. Same thing if the prosecutor knows of evidence of innocence and withholds or, or if the evidence only turns up after the trial. You get only one chance and then you’re screwed.