alt Hacker NewsI say "simply" because, as I mentioned, it's an invariant—possibly the most consistent phenomenon we've observed on HN [1]. I admit I didn't add any substantiation for this! it would have been too much of a digression. (Not that that ever stopped me before; perhaps I'll make up for it now.)
The interesting question is, if it's so consistent, how can it go unnoticed for so long (as you've reported) and/or get perceived as one-sided ("HN is so anti-A!") when in fact it is almost always two-sided ("HN users are divided on A")?
I believe there's a clear answer: it's because what you notice depends on how you feel [2]. If you like A, or (more precisely) dislike anti-A, you are far more likely to notice anti-A posts. Not only that, but they will make a stronger impression on you than the median post—even the median pro-A post.
These two factors—frequency of impression and impact of impression—combine to create a strong picture of the site as anti-A—so strong that people often use universals like "always" and "never" and "absolutely" when describing it. In reality, HN is a statistical cloud of datapoints and your pre-existing feelings are what determines which datapoints you notice (i.e. frequency) and how strongly they affect you (i.e. impact). [3]
This is why people with strongly opposing views feel the same about how biased HN is, just in opposite directions! i.e. with the A team convinced there's a strong anti-A bias and the B team equally convinced there's a strong anti-B bias. It's simply (<-- that word again) that their feelings cause them to notice different subsets of the available datapoints. Abstract out the directional bit (pro- or anti-A), and their comments become isomorphic.
Unfortunately for us, HN is more afflicted by this phenomenon than sites which allow sharding, whether by social group (e.g. Twitter's follow lists) or content (e.g. Reddit's subreddits), and thus organize the community into silos [4]. HN is non-siloed, meaning everyone is in the same big community: all the As, all the anti-As, all the Bs, and all the anti-Bs - all roaming the same threads and bumping into each other. You are therefore more likely to run into datapoints you dislike and so more likely to feel that the community is biased against your view, and - what's worse - more biased the more strongly you feel!
For me this is sad, because I believe that in reality HN is a somewhat (<-- let's not exaggerate) more thoughtful and tolerant community than most others of the same or greater size, but precisely because of this dynamic, ends up being perceived as less so. (I wrote a thing about this a few years ago, if anyone is interested: https://news.ycombinator.com/item?id=23308098.)
I believe this is why one so often hears about how toxic, nasty, negative HN is—not that it isn't those things! but the relative level of them gets distorted. Human beings simply can't take very much of what they dislike and disagree with before resorting to generalization and other internal barriers. This is basically an immune response. It often takes only a handful of datapoints (3, or 2, or maybe even just 1) before the impression burns into the retina and becomes a permanent image [5].
Side note: in one case I saw, some people were agreeing about how terrible and mean Hacker News is, and to prove the point, one of them linked to a vile reply he had received. That reply, however, was from Twitter—not from HN at all! He hastened to add "HN is the same"—somewhat self-refutingly, since if this were true, an actual example would not have been hard to find. In reality, while such vile comments do appear on HN, the community quickly flags most, and moderators eventually flag most of the rest. This is an example of the skew in perception I'm talking about, and while I admit it made me feel terrible for the better part of a day, I don't mean to be critical. I understand where it comes from, and it comes from fundamentals we can't control.
This phenomenon is at its most painful when the topic is close to one's heart—for example, when one's own work is being discussed. In cases like that, it doesn't take much before one feels wounded, and such painful impressions rarely go away.
Once or twice a year, some reply I'm writing turns into a long digression about this because it makes me sad, especially since, as I just said, it comes from fundamentals [6] - in this case, how HN's design interacts with human hard-wiring. As a result, it's not going to change, nor can it be affected by argument.
Or rather, it could change only if we changed the foundation of HN's design—in this case, by sharding the site into silos—but (a) I'd be scared to tamper with DNA at that level, and (b) if the above argument is correct, it's actually good for the world that this place exists; it just can't expect to be perceived as such [7]. Thus ends this season's lament.
[1] https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
[2] I don't mean "you" personally, of course—everyone does this. It's a double whammy of https://en.wikipedia.org/wiki/Negativity_bias and https://en.wikipedia.org/wiki/Clustering_illusion, sometimes described in this way: https://en.wikipedia.org/wiki/Hostile_media_effect.
[3] Lots of past explanation here if anyone wants it: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que....
[4] https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
[5] https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
[6] https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...