Due process was given at trial. Reasonable doubt was tested at trial. Every appeal has re-examined the issues. Minds can differ.
I base this history and personal experience.
Eyewitness testimony is infamously unreliable. There was no other evidence in this case; no DNA, no physical evidence - NOTHING.
This was a high profile case an it is very possible that the prosecutor wanted a conviction in the case more than he was interested in convicting the right person. After all this time, you think a prosecutor is more interested in admitting there were mistakes than in doing justice?
There are many instances when in the face of undeniable proof of innocence, a prosecutor will still claim he guy was guilty. It happens all the time.