Maybe because of events like those in this list: http://geekfeminism.wikia.com/wiki/Timeline_of_incidents

The same reason there are so many more male geeks than females. Biology and society.

It would help tremendously if more women would stop asking this question; stop asking for permission to participate, stop asking for approval. If you want to hack on something, go out and hack. You are a free woman.

This place is too crowded with men, I guess.

Why are there so few female car mechanics? Same problem.

Because hackers and coders use rude/inapropriate/discriminatory language and expressions, behaving often as in a adolescent group of boys.

They get intimidated, it's hard to develop in a strict language, they are waiting the next version of lisp-ada which will produce any program from a dialog with the computer.

They get frightened.

Good question. Help us with this please!

Because not enough efforts are made (yet) to welcome them!