Because trials are infallible no doubt.
The problem there of course, being that prosecutors are rewarded for successfully winning cases. Alan Gell was accused of murdering someone in 1995. Fortunately, NINE years later, it was learned that prosecutors withheld key evidence that they had found, because it weakened their case. He was acquitted in 2004.
In your world though, Gell would have been dead long before this revelation occurred.
EDIT: Link
Turns out in that same article, a 1984 man convicted of rape was released a few weeks before Gell was, as later DNA evidence showed he could not have been the rapist. That one was 20 years after his conviction.