"Why aren't there more female bricklayers?"
a) because bricklaying (and construction jobs generally) tend to be physically demanding, messy and dangerous, and women generally are less physically strong and less willing to take risks / less stupid (delete as appropriate) than men.
b) because construction industry workmen tend to be very sexist (at least in the fairly small sample size to my knowledge)
Reason (a) and similair will exist for many jobs where more women or more men will be more attracted or less attracted to certain jobs. That is natural and choice of jobs that follows is to be encouraged. Reason (b) exists in many other jobs (eg traders), of which IT can be one, and is in any case to be discouraged