Because it goes against the results they wanted to arrive at.
In comment after comment, the possibility that there is no bias is considered literally impossible. The blatant and unrecognized bias in this conversation is absolutely fascinating. (I also notice HN has introduced a throttle, presumably to reduce the volume of incorrect messages.)
That said, it's not super hard core militant censorship, there's plenty of other people saying politically incorrect things in today's discussion as well, so my censorship may have been due to "not adding to the discussion in a meaningful way", rather than my particular point of view. Hopefully there's an appeal mechanism for when I decide to straighten up my act, I'm working on it.
But as they say, freedom of speech does not require someone to provide you a stage to speak on, and private companies are free to censor whatever speech they see fit.
It might be depth within a certain timeframe then. I've noticed similar delays previously in non-controversial topics when trying to reply to non-controversial comments.
I don't think there's anything sinister going on here, there's just a mechanism in the comments to prevent people from speaking past each other in quick succession.
Edit: For reference, this comment took 9 minutes before a reply link appeared, and it is the 9th child of a top-level comment. Coincidence? Maybe, maybe not. Try replying to this comment and timing how long it takes before a reply link appears - my guess is 10 minutes.
An alternative explanation is the universe was punishing me for being a dick, which wouldn't be that unrighteous.
If you can reasonably assess that your process is fair, I don't think anyone should care how the numbers bear out.