Why do valid posts like these appear 'dead'?

Different reasons, but if it's a user comment it's probably because the user has been shadow-banned for violating HN policy somehow. If it's a top-level post, it could be that the domain has been blacklisted for contributing too many low quality / spammy posts in the past.

And, I assume the HN mods have the capability to mark any post or comment as dead if the need arises. I'm not an insider so I can't say that with 100% certainty, but it appears to be the case.