One possible benign reason: as society has gotten more free and open to women's choices, their preferences have been revealed. (Note that this also may explain why there are more female computer programmers in Iran than in, say, Finland. Women are more free to do what they want to do in Finland.)

Now, look, this explanation may be wrong, but, no, it's not hard to imagine alternate explanations. Nice and neat conclusions in the social sciences are hard to come by. Anybody in this thread claiming that they know exactly which explanations are the right ones is full of shit.

I don't agree that this is a benign reason in itself. Regardless if you say that women don't want to be programmers now or that women never wanted to be programmers, you have to find a good reason why they don't want to. Because women have no problem sitting in front of computers in economics, so why do they in software?

I don't think it is hard to find a good reason why someone wouldn't go into nursing (even if this wouldn't disprove sexism in nursing). It has barriers to entry by requiring formal education, which can be competitive. The working conditions are somewhat difficult having to work irregular hours and be exposed to emotional stress as in seeing people dying. At the same time it still relatively low status, not necessarily well paid and has limited career prospects.

Of course you can argue that this shouldn't be the case. That the profession should help people dealing with stress and have good working conditions etc. Still it much harder to imagine these sort of "hard" reasons, negative things that are inherent to the profession, when it comes to programming.