In the early days of jetways, they were built straight out from the terminal and the planes would pull up parallel to the terminal. If the plane was long enough and the space was available, they would use 2 jetways-one for the front and one for the rear.

As airports started to become more crowded and more gates were needed, someone had the bright idea to turn the planes perpendicular to the terminal with the jetways now able to roll up to the side of the plane. Remember, it's more about utilizing the limited real estate the airports have to move as many passengers and planes as possible in the quickest time possible.