This varies from country to country and is also a matter of intense debate, at least in the United States.
One possible reason is discrimination. It could be that the men (largely) who make the decisions on hiring and promotion simply do not believe that women are as capable as men. This may or may not be a conscious belief, but you can argue that it stops women from being hired for or promoted to top positions.
But you can also argue that it is not actual discrimination that is to blame. Instead, you can say that it is the fact that our society expects women to interrupt their careers, at least temporarily, each time they have a child. These interruptions, even if brief, can disrupt women's progress up the corporate ladder.
One of the major reasons as mentioned, might be discrimination. A woman in power is still often frowned upon by her less superior male colleagues. Lets not put all the weight on discrimination. Complaceny might also be one of the contributory factors. I am not sexist but most women tend to want to stay at home and look after their children. As a result of which, there are less women in the field of work. And hence less women at high management posts.