Network-effect makes a big difference too. After all - you have to arrive here some way - usually by being told about it by friends. Sure, some people arrive by accident - just happen to be browsing through HP fanfic or something... but a lot will arrive through their friends... and a lot will stay because they find friends.

...which leads us back to people's friends tending to be same-sex. If there are few people of your own sex in the group then it's got less... er... ambient friend-potential...

you have to work harder to be with a bunch of people that have a different culture than yourself. Genders have different cultures, so add that on top of the new culture of LW itself and unless you're a particularly socially-capable person (and LWs tend not to be), then it's less likely that you'll find friends.

Obviously this is a generalisation and likely only a very small part of the pressures involved in a very complicated process... but it's there.