actualy ryanvdonk is correct.
Girls tend to have less problems with tip dive due to where their natural center of ballence is.
Rocker like all ski tech, has it's advantages and disadvantages. The obvious advantages of rockered skis are that they float easier and allow the ski to turn quicker. Girls tend not need the rocker, for flotation (its biggest advantage) as ryanvdonk said. Also due to the geomotry of Women they tend to have a much easier time turning skis then guys do, so the other advantage of rocker is not necessary. Rocker does have disadvantages, these include lack of edge control and stability. The two places a female geometry does not help. due to their smaller size and different center of ballence Girls do not have the natural stability when going fast as guys do. Nor are they able to preasure and hold an edge as well as guys are. Putting rocker on a womens ski would make these problems worse. Since their weight is naturally further back then it is for a guy, imagine how little stability they would have on a ski with tail rocker. The likely hood of falling on their ass would go up tremendously. Rocker also reduces the amount of edge contact between the ski and the snow, This would be considerably more hinderence to girls then it would be to guys.
So basically: The benefits of rocker are not as helpful to girls while the downsides of rocker are more of a hinderence then they are to guys. combine that with the fact that the market of girls who would need/want a rockered ski is much smaller then it is for guys, and many rockered skis are soft enough for girls and you get why there is a lack of rocker skis marketed towards girls.