The Space Shuttle was expensive and this was apparent even at the end of the design stage. It was hobbled by the partnership with the Air Force, where it needed

The Space Shuttle was expensive and this was apparent even at the end of the design stage. It was hobbled by the partnership with the Air Force, where it needed a large minimum payload and specific dimensions for the cargohold. Design considerations and technology limitations also made it not very reusable. It was expensive as a human and cargo launcher and eventual savings from learning (many launches) weren't feasible to begin with. The Air Force realized their mistake and went on to develop their own separate launchers. The Shuttle continued because it had acquired momentum and had large constituency support, it was a jobs program. Nixon also favored doing one thing over and over (i.e. Earth orbit) over potentially more risky one-off deep space missions. The ISS was eventually pushed through as a way to give the Shuttle something to do, absent any other US deep space aspirations. Mars Direct lays the trail for a continued program of exploration, not just a 'footprints and flag' operation. It has interesting savings and synergies even without necessarily having reusable launchers (except for the NERVA-style hopper). Technologically it's also very conservative.