It's way overspecced, because the DoD wanted the ability to go into very high orbits(which they only did once, to place a satellite, I believe. Could probably have been done cheaper with a rocket) and even enter polar orbits?
(Not certain about that last)
Those demands required a larger fuel capacity, larger, heavier engines, a bigger and heavier shuttle in general.
It also resulted in them having to strap on the solid rocket boosters. Something that should NEVER be allowed on a passenger-carrying craft as they don't have a shutoff!
Did you know that the Russian 'copy' the Buran(Translation: Snowflake) could land by autopilot?
Something that was never possible with the NASA shuttles.
The shuttle is like an off-road version of an RV.
A big, fuel-guzzling monster with capabilities almost never needed.
It would have been cheaper to design a few special versions for those 'offbeat' missions and leave the shuttle as was supposed to be; an efficient (well, compared to the Saturn V) orbital transport.
Imagine a sleek passenger version without the enormous cargo hold, perfectly suited to ferry astronauts to and from orbit.
A 'Heavy lift' model with a cargo bay and a smaller crew compartment(pilot(s) and mission specialist)
A 'high orbit' model with smaller cargo bay and detatchable liquid-fuel motors...