I think it is because it is harder to fit text on a vertical bar. It has to be rather wide to fit a couple of words horizontally, and even then the text will of

I think it is because it is harder to fit text on a vertical bar. It has to be rather wide to fit a couple of words horizontally, and even then the text will often get cut off. The text could be rotated to read vertically, but that is hard to read.