An interesting topic. Here are some of my ideas that have not yet been addressed in the post or this thread:
What if we look at the possible causes from three points of view:
1. Why don't women run more?
2. Why don't voters vote for the ones who do run?
3. How does the American system affect this all?
As far as I can tell, most of the answers so far are about why women don't run more, and a few are about the system and how it affects that.
My angle is slightly different as I was born in Europe and have noticed that there are very specific differences between the American system and some European systems, especially those of Scandinavian countries, and these differences affect the chances of women succeeding.
These aspects of the American system also interact with the latent distrust of women as politicians in many voters' minds, and it is this that I believe causes the scarcity of women in positions of political power. It may even feed back into how women think about a life in politics and may explain why so few choose to run.
A short summary of my theory is this: The two-party system with the winner-takes-all aspect means that it is risky to run a woman against a man if the country is at all sexist against women. The sexists will not vote for a woman, but the nonsexists will vote for a man, so choosing a man is always safer. Note how low the female representation percentages are in all the two-party countries. I believe that particular arrangement hurts women and also minority candidates.
It is also risky to elect a woman as the candidate if the stereotype of a politician is a Teddy Roosevelt-type person, and if the candidate is supposed to have single-handedly strangled many enemies in a war.
It is risky to elect a woman when the language of politics is all about baseball and wars and when aggression is the one thing that seems so very lacking in women. Though I would have thought that we have learned a lesson about unthinking aggression in the White House during the last six years...
Now, the last two paragraphs don't describe sexism as such, but they do describe a system in which being a woman can be a handicap. Of course being a female politician can be advantageous, too, if women practice reverse sexism and are more likely to vote for women. But it is unclear that this would happen enough.
To return to women's reluctance to enter politics: Note that women are not equally reluctant to do this in all countries of the world. This suggests that it's not just some innate gal-thing we are talking about here. Perhaps American women know that they will be put through a very special wringer in politics. Their worthiness as mothers will be questioned in a way that is not done with men, and even their ability to be good wives will be publicly doubted. Just imagine if Newt Gingrich was called Nina Gingrich but with Newt's actual history. How would this Nina have been treated by the media and the American public?