Because substantial segments of the community hold deeply-held, diametrically-opposed views on the issue and do so as a matter deeply attached to their personal identity and person image of merit.

For better known examples of this in general (US) cultural, consider debates on guns or abortion policy.

(If you want to know why they get flagged off heavily, part of it is that -- since there is a large group that will be offended by any item on the subject, though a different group for different items -- and partly the reaction of the rest of the community to the usual dynamics of the issue where they just want it to go away.)

Personally I find that HN is a nice place to get away from the non-technical side discussions that take up so much space on Reddit, Google Plus, etc. Discussions about sexism fall into that category for a technical site (elsewhere they would certainly not be side discussions).

While they're important, I have other places where I know that non-technical discussions will include those topics (and they often do on a daily or weekly basis). In the end, a site about technical and technology news should avoid controversial topics, much like a political site should avoid technical topics.

Just my $0.02.

The term "sexism" is inherently political. The political factions on either side of the divisions for which it proxies have well stocked and battle tested arsenals and fervent internet hoards with their itchy trigger finger already on the keyboard.

If the HN community is a reflection of anything in the real world, it's because people are sensitive to personal critiques and sometimes react by immediately assuming an intensely self-defensive posture, even if they reflect more thoughtfully on the critique later on.

Not everybody does that, and few people do that all the time, but it doesn't take all that many people for it to lead to some heated discussions and wild swings in up/down votes.

Thanks to the OP for asking; I hope there will be a real conversation about this. I've been noticing that often posts about sexism or feminism start to get some traction and then inexplicably disappear or are nuked, and it's super frustrating. How can we have discussion if posts that might encourage it just disappear?

The posts disappear quickly because, by and large, the community here doesn't want to have a discussion on that topic. (And when it does, its very often not a productive discussion. Too many sacred cows, on all sides of the issue.)

Understood, and I agree that often there isn't very much productive happening by the end. Hopefully that will eventually change.

Certain topics are taboo like Gamergate or Sexisim in STEM.

They lead to flamewars and bring in trolls.

Sure we all want equal pay for equal work and more diversity in STEM, but these sexist issues just bring in the SJW trolls and then the topic goes from bad to worse.

It is something that shouldn't be discussed on Hacker News, because it is more political than technical.