I can't say I know the cause, it might be social encouragement (or lack of) and/or natural tendencies, as a guy I was building radios and other electronic devices with my friend and then went out and pranking people who were playing radio too loud, somehow I couldn't see my sister interested in that kind of pranks. Girls mature faster, that's I think at least one non-controversial statement. Building and playing with devices and programs might require a non-mature type of mind that boys have for a longer period, girls might not be so interested in this kind of stuff once they pass a certain age, I wonder if more girls would stay in the field if they are involved at a younger age into these kind of things. I can tell you that when I was 12-14 or so I was fascinated by pong and was interested to write a pong program, again, when my sister was 12-14 she was probably bored to tears by pong and she wouldn't even dream of wasting time to write a pong program.