Maybe because of events like those in this list:
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!