Because men's game has more deapth and variety. Women's game revolves around sheer power. It seems the more power a player possesses, the better she is in the rankings. In other words, no matter how skillful the player is, as long as she lacks in power, there's a limitation to how far she can get. On the ohter hand in men's tennis there are crafty players like Santro and Coria who, despite limited physique can match big guys.