it is a very odd thing... and i do have a somewhat strange reason which may or may not be true;
you kind of have this exact thing for any kind of subculture. you don't get many female metal-heads going to festivals, you don't get many female rappers or going to rap shows, you don't get girls going to re-enact viking battles or anything like that. i think it has something to do with identity, hence why you don't see many black guys at metal shows and things like that.
i know it's a very bizarre assumption, but i don't think that women feel so much of a need to cling onto an identity, because we live in a male dominated society.
who knows... this is just something i've created with my own imagination based on very loose information.