Ever notice that all of these exculpations are two or three decades old?
And that they never occur in a case where there's an active, determined victim group. The Troy Davis case is what happens when the victim is active. There, the defendant argued aggressively for more than ten years that the case against him had come unglued. Finally, the Supreme Court ordered the hearing he desperately sought - and it turned into a complete cluster f--k, because his defense couldn't begin to prove what it had been claiming. I guess the witnesses had talked cheerfully enough, in an office with sympathetic listeners or over a nice steak dinner, but they were absolutely unwilling to do so in open court. The hearing was a farce. That didn't stop Amnesty International from issuing press release after press release to the effect that 7 of 9 witnesses had recanted. It wasn't true, but that was a matter of indifference.
I'll clue you. There's no such thing as a perfect trial. Discovery requirements are onerous, and in the vast majority of cases there's a failure somewhere. That's as true of big civil litigations, conducted by the huge shops, as it is in criminal prosecutions in local offices. It doesn't have to do with Great Evil, but human fallibility and the fact that clerical work, easy in concept, is actually very difficult in practice. Since no one I've ever met really wants to convict an innocent person, and if you've got the right guy, most 'exculpatory evidence' can be blasted into smithereens (the crime either happened or it didn't. If it did, there isn't any exculpatory evidence that actually exculpates, properly explained in context), most discovery failures are inadvertent and harmless.
Which is the reason all these cases are so ancient, and/or unguarded. When the case is live, someone is going to dispute vigorously the effect of the newly discovered error, because he or she really doesn't like being slandered . After 20 years? When everyone associated with the case has disappeared? And right or wrong the inmate has probably done enough time? Suddenly the missing piece of evidence is exculpatory - the critical missing piece - has to be to get you out of jail - and the (always nameless) prosecutor is malicious. The various state AG's should do a better job on these, because the release doesn't matter so much, but the engendered cynicism is not.
Mind you, a system as rich in human reality is obviously rich in human fallibility. Of course, error occurs - big, horrific error. But not with the systematic malice that the the Innocence Project(s) proclaim. If you attend court, in the ordinary case, you will see people who come from the same culture as you do, with the same notions of honor and fairness, the same openness, doing the best they can. Prosecutors sit side by side in the same law schools as others. They get into the business because of the same notions of justice you have. Ditto FBI agents and the police - some - most, i think - will impress you with their conscientiousness. Others not so. Of course, they make mistakes. But they're not a different species.
What should be happening in these cases? No exonerations, thanks, unless the defendant submits a detailed description of the theory of innocence, an alternative explanation of the incident that can be tested against all the evidence accumulated in the case. An exoneration does NOT occur because a scrap of missing evidence, taken in isolation, might conceivably have caused a different result. The reality is that there is no fact that occurs in isolation. Context is everything. A defendant is not exonerated without a demonstration that the entire context is different.