The answer, I believe, lies in women's mostly non-confrontational attitudes. They avoid direct confrontation at all costs as a general rule. I think the psychology of this goes far deeper than I want to go into on a forum, but I think it has mostly to do with the sense many women have (whether natural to them or learned) to please everyone and consider everyone's feelings.
The counter to this is perhaps more enlightening. Take Clinton for instance. Here is a woman who is strong and opinionated, and she isn't afraid to show it. This gets her labeled as a bitch by many of the aforementioned types of women (and even some men). She has stepped out of the narrow confines of the historical role women have been taught to play in this country. This makes her a "feminist" in the real sense of the word. She is not a woman fighting for women's rights so much as she is a woman who has broken the stereotypes.
Palin, on the other hand, is completely wrapped up in the soccer mom stereotype and seems to be proud of it. She shouldn't be. This stereotype is vapid, conniving, and completely cow-tows to the fifties "Leave it to Beaver" archetype. It is everything I loathe in women!
