Because most of the people think that men are more talented than women and are more capable to work than women.
even here there is the hand of backward thinking in the people that the women should stay at home and do all household works. they think that women shouldn't work in offices or work as jobbers etc.
even today crimes especially rapes kidnappings acid attacks have increased in number. That's also the cause that people don't let girls or women to work in offices etc.