Few things:
1) WTBTS teachings do not appeal as much to people with higher education. Women, historically, have been less likely to get a higher education than men.
2) WTBTS ties you to the org with 'emotional blackmail' (IE., cutting off family, friends, etc, if you leave). Men are, traditionally, much less susceptible to this.
3) WTBTS doesn't really offer much other religions don't have other than an unusually close and closed community - promise of meeting up with dead friends and family, as well. Very tempting to women, not as much so for men (generally).
The org is really tightly focused on controlling its women members with its specific policies. So, why are there any men at all? Well, it has something for them, too. Power. In the org, men have and can get power over others - and, women specifically - fairly easily.
Only problem for the org is, as more education is achieved, the seduction of raw power for its own sake becomes less tantalization. Alternatively (for those to whom the need to feel power over others is strong), there are much more significant ways to attain power in the world than the faux-control the org offers.