to answer your question simply and clear, you have to know that Allah allow and prevent things basing on what is good for us, its not just random rules.
 The woman does not lead others in prayer, which means she can not be 'Imam', because she is not all the time ready (Women does not pray during her period/after giving birth ect ..), so for this reason, she cant lead, and once she cant lead, means she will be led by the opposite gender, so its logic that women will follow them by being after them during salat, and its because males and females does not mix together, and for a woman to watch a man praying in front of her, does not harm as if a woman pray, bow and prostrate in front of a man, ... so for this reason, women stands up behind the men during Salat, and basicly at any gathering.