As best I can tell, it started as a bunch of men in a male dominated culture at a time when it was mostly men posting online. It is a public forum. Women online seem to gravitate to more walled garden type environments. Women online often post anonymously. This forum has a professional emphasis and even men who post under a handle instead of under their name are often pretty open about their actual name. A lot of men at the top of the leaderboard are people whom you can find articles about or a page on their employer's site or something like that. So there is third party corroboration of their identity. This is something women seem to be less likely to have in the first place, and it was worse some years ago when this forum was getting formed.
Each of those norms are norms women have trouble meeting. The more individual things you add that are hard for women to meet, the more excluded women are.
It's a little like saying "You don't have to be male, you just have to be over 6 feet tall, able to bench press X amount of weight and have a beard." Even if they decide that having a beard is some arbitrary sexist requirement that they are willing to drop, you still will find very few women who can meet the standards required to participate.
(Disclaimer: I have barely slept and posting here at all is probably a bad idea on so little sleep.)
1) it's a male dominated culture
2) people somehow communicate they are men
3) women prefer posting anonymously
Interesting... and how about the news that get posted, do you think they read them but they just don't comment?