It's all about computers and interwebs and programming and that stuff, this still is a male dominated sector, I studied computer/software engineering, something like 100 guys and less than 10 girls entered per year, I don't suppose that has changed...
Look at all the geek/nerd stuff out there, Linux, file-sharing, gaming, you don't see many girls.
Yes, it's all very male-dominant. I don't believe males are pushing women out of these circles either; most of them seem more than open to women. Of course you have the off bigot but for the most part they're more amazed a lady could be interested in such things than against it, so I think that can be ruled out. There are also scholarships open specifically to women by various organizations to get them into technology, so I don't think that's an issue either. My next guess would be societal norms; I can only speak for America, but it's a common (yet diminishing, especially if we look toward the younger generations) stereotype for ladies to be either subservient to men, or more interested in "womanly" work; fathers want to teach their sons leadership roles and such, but rather the mothers teach their daughters more subservient roles--after all, that's what they learned, and that's what they want to pass on.
Most of the women I've known have had an incredible interest in nursing; this is strongly tied to empathetic responses to the injured and sick, where women are oft more empathetic toward animals and other people than men are. At the same time, jobs in technology are more "cold", and often not oriented in the pursuit to directly help people (at least, not in the same way being a nurse would; of course technology is helping us greatly, but it doesn't have that immediate response sensing personalities need to feel they're doing something well.) The biggest question I have here is whether or not this is something woven into female genetics, or something beaten out of males, or something nurtured in women--perhaps both. The existence of people who feel great empathetic responses to harming animals but give fuck-all when a person is killed seem to support this idea; if you can train a person to hate their own kind, is it possible to shape how empathetic a person can be while growing up? Is a person's personality set in stone the moment they are born, or is it something which is fashioned? Evidence seems to point to the former, unfortunately; a person who goes through great traumatic stress can still be a loving, caring person, and a person with a wonderful childhood can still be extremely ordered and authoritarian.