Because we get flagged down a lot. This thread managed to survive because we started a big twitter stink to try and get it unflagged, and because we had the international women's day hastags to get attention, it managed to make the front page. But even that was a struggle.
Many of us, myself included, fight on behalf of a lot of underrepresented groups on HN as best we can.
I'm sorry if you haven't seen it, but I assure you some people here are trying.
I see it come up pretty regularly. It usually is then followed by racist pablum of one sort or another--so the resistance to racial issues isn't exactly different from the resistance to gender ones.
Because we have enough difficulties discussing basic issues of gender equity, without trying to deal with intersectional and allied issues. Walk before you run!
Why don't I ever see it being discussed on HN?