Most of it comes from a series of awful compromises with the military. The Air Force wanted it that big so they could put up and recover very large classified c

Most of it comes from a series of awful compromises with the military. The Air Force wanted it that big so they could put up and recover very large classified cargos from various places, iirc in less than one orbit. Also, if the shuttle had actually been used as much as intended the cost wouldn't have been nearly as much.