I'm not going to speculate too much on reasons - anything I might say would be conjecture, and no doubt offensive to one party or another.
One thing I will say is that on any forum the percentage of female members who are willing to openly declare themselves as such will always be below the percentage who are female: many females don't want the sort of patronizing attention that comes with having their gender known, especially in what is perceived as a male-dominated field (cars, computers, really anything 'nerdy'). They tend to be treated quite differently if the other members are aware that they're female, and a lot of them just want to be treated like anyone else on the forum.
I will say that in my family the females are by-and-large as interested in and appreciate high quality audio as much as the males. Everyone in my family has a pair of headphones of at least mid-fi quality, although my sister and I are the only ones with dedicated amplification. So I don't think females appreciate hi-fi any less than males do, in my experience.