How many times have we seen people who just can't get anywhere with a judge to have a death sentence commuted to life, just because the judge wouldn't allow new evidence that, while it may not prove innocence directly, does support the existence of another possibility?
There is a bias that the judges and prosecutors, and police for that matter, have, that precludes the possibility that they could be wrong. They just won't consider it, unless it is something so big they are incapable of ignoring it anymore.
They just have to be right. All, the time. This kind of mentality, pretty much assures, that innocents have been executed, because, to even acknowledge that they could have been wrong in even the slightest degree, then they have to also deal with the reality that they have killed someone who didn't have it coming... and face the scrutiny that comes with it.