For the shuttle there was also a lot of interference from the Airforce, which forced them to have a massive payload bay, so AF could launch and service their big Keyhole satellites (and possibly snatch Soviet ones in one orbit without warning).

Then there was pressure to make use of solid propellants to amortize their use with military uses, not to mention pushing for Shuttle carrying all payloads for the US.

This together created the dangerous and expensive design tHat was STS.