With a lot of modern bikes, the top of the seat post is kept low so short people can ride them. Tall people can use a long seat post and ride the same bike. So manufacturers make bikes to fit a wider range of people, instead of having a wide range of frame sizes. In the past there were a wider range of frame sizes. This might not be true for all manufacturers.

In the old days, there were boys bikes with high bars and girls step through bikes. Step through is ideal for girls wearing skirts or dresses. In Asia it is still common for girls to ride bikes to school or work. Having a low bar, means the same bike can be used for boys and girls, instead of having two different frames.

Having a lower bar makes it easier to get on and off.

A horizontal bar may have a very tiny aerodynamic advantage.