I'm surprised nobody speaks of children and pregnancy ...
In other fields, this is why a man would be preferred to a woman, and this is why, too, for instance in France - at least, women earn 20% less than men for the same position say the statistics ...
We live (at least in western and Christian countries) in a patriarchal society. Women really started to work in the very late 19th century (for the most industrial countries), and until my grand-mother generation, women were the ones taking care of children and the social life. They could have hobbies, but these were not taken seriously.
In arts, women started to get reconnaissance in the mid 20th century - even though there were Marie Curie, Camille Claudel and Colette, those were exceptions -, in the postwar period. I'm not a specialist of this, but who can give me more than 10 names of female photographers before 1945 ? Moreover, from its invention til the first three decades of the 20th century, photography was not considered art and was, too, a very technical medium, big materials to carry etc. and the women were the ones in front of the camera mostly, not behind the lens ... These are only cliches, probably ... And there's probably some women photographers, amateurs, we don't know yet about ... But, I'm not sure that in Europe, there were a lot of those women.
So women don't lack of talent, for sure ... There's just no big and huge and tons of them before the generation born btw late 1960s and now ...
And I think it's true that their subject and complexity of work is very different from the men's ones ... Probably, because they don't want to repeat the machist thematics of sex ...
Why do people hire men to shoot campaign ? Is it because they are better ? I doubt ... It's because these two current generations of photographers working (Testino, Teller, Mondino, Richardson etc.) were not yet generations where women were yet dominant ...
And perhaps, because women start a career in photography, but then they have children and a family and take care of them ... So they focus less on their careers. You know there are still a lot of pressure on women who do not have babies and family ... I know very few women who focused on their careers over their "womanly expectations" ... Those manage to be better than men, most of the time ...