womens bikes are made with lower bars because of the fact that some ladies used to wear skirts while riding.
The first poster is absolutely correct. When the bicycle was first invented all women wore long skirts/dresses, and they had to put their leg through the V of the frame, whereas men wore long trousers and could easily put their leg over the saddle, many women still wear a skirt or dress on a bicycle, so the design remains basically the same.
In modern bicycles, there is no difference between male and female specific designs other than adjusting the sizes to accomodate the general differences in anatomy. I haven't seen a woman riding a bicycle in a skirt or dress in many, many years. The manufacturers who are still making the old woman's design are doing so for marketing reasons, to have a retro-looking product.
Because it is easier for a woman to ride a bike while wearing a dress or skirt. Most women don't do this anymore in America, but they still do in countries where bicycling is used for transportation more.
This question has been asked once a week forever here. And stated in almost the same manner.