It's great and important when an innocent person is freed from prison. Are you the attorney described in the LA Times article? If so, that's amazing and I'm sure everyone here will warmly applaud what you did.
The HN aspect is strangely beside the point, but maybe I'll address it, since it might help in future situations. Condemning the community seems a bit unfair. One need only look at the top comment in the earlier thread to see that 'nobody took the time to read my letter' is false. The problem was that the post was flagged by other users.
Rallying readers urgently to a cause is a hit-or-miss endeavor on HN, and the way you brought it up likely made it hard to distinguish from the run-of-the-mill politicized stories that appear every day and get flagged every day. On HN, an urgent tone turns out not to be a great way to represent an urgent case—even a genuinely urgent case—because countless stories take an urgent tone and they nearly always turn out to be crying wolf, overselling, or misleading. Readers have built up antibodies to that, and many come to HN to escape it.
Had you asked us for advice (which people are welcome to do by emailing hn@ycombinator.com), we might have suggested that you post an Ask HN and explain that in addition to being an HN user, you're an attorney who has discovered a miscarriage of justice, and would the community critique a letter before you send it. That would have been different in two important ways: you would have engaged readers' intellectual interest, and you would have established bona fides as a community member and an attorney. By contrast, trying to stir up interest Reddit-style tends to backfire on HN. And comments like "What we're dealing with is essentially the classic deontology versus util debate" probably took the thread in a direction that diminished its credibility.
But all of that is nothing compared to correcting a profound injustice, and if you did that, deep respect is due!