This particularly refers to a job or position that has been filled by one gender. According the US Department of Labor, non traditional jobs that have one gender has less than 25% people employed in each position.
The position or industry in question simply might not hold as much personal interest for as many women as it does for men.
However, many people argue that there are also regrettable situations in which those responsible for choosing new employees have either a bias against women or have an unwritten directive to give preference to male applicants. This ambiguity has prompted much legislation and investigation in the modern economy.