It was designed by politicians not real rocket scientists. Real rocket scientists wanted to put a propellant depot in space, use re-usable rockets, modern engines, you know, the things that would drive costs down. The politicians had no desire to drive down the costs, they only wanted money for their districts. So they built a rocket with no clear goal in mind (remember the "asteroid redicrect mission?), just keep the space shuttle economy going by re-using space shuttle hardware. Except the SS was re-usable, except for the tank.