Note how they were exonerated while still on death row, not after being executed, in which case the death penalty is irrelevant. Furthermore, someone on death row gets extra appeals, which means that many of these people would end up serving life in prison instead of being exonerated.
Ah, so there are actually people who work to exonerate people post-execution?
When the stars threw down their spears
And watered heaven with their tears:
Did he smile his work to see?
Did he who made the lamb make thee?