My answer, originally published on Quora.

My Answer

The most obvious answer:

  1. Fewer and fewer girls are studying math and computer science in college, and
  2. Of those women who do graduate in those fields, and who even become programmers, they are likely to leave the field after a few years.

Others have addressed #1 in other answers. I'll address #2, as a former software engineer.

When I was in my twenties, I loved programming. I was very into the tough problems that earn you nerd status and all that. And I really enjoyed working with all of my engineering peers, 90% of whom were men. I was good at programming, was well respected by all my peers (male & female), and got to work on great projects.

So what happened? Well, the Sales & Marketing people disproportionately wanted ME to be the technical person who they took along when visiting the "tech guys" at the customers and prospects. I was referred to as "Nerd Bait" by some of our company's sales people because I was young, reasonably attractive, and loved talking shop with customers.

From there, as a tech person who was always visiting customers, it was only a short leap into Product Management, which I still do today (although I do miss coding).

I've seen the same thing happen to several very competent female engineers, where they ultimately end up in Sales or Marketing or Product Management.