It's because kava producing countries are largely 'third world' countries, who for the most part still have a strong attachment to ancient village life and traditional rules. Primitive cultures around the globe typically held the belief that men do men stuff and girls do girl stuff. Fragments of this mentality persist even in our modern culture, but is much stronger where life is still traditional.
In Vanuatu, Port Vila has been modernized enough that women will publicly drink kava, though still in much smaller numbers than men. If you go to a more primitive island or traditional village, they still hold the belief that it's Tabu for a woman to even look at a nakamal when it's kava time.
Hawai'i, of course, has assimilated to modern ways so much, that it's not really an issue there.
There seems to usually be about 4 or 5 active female members on the kava forums at any one time, several other previous active members and certainly many lurkers. We've had a thread on it before, but it seems like there's an inherent divide, even when you have no attachment to traditional rules.
In my life, the only person willing to drink kava with me when I bring it up, is a woman. She still doesn't prefer it though.