It never could! In order to get Nixon to sign off on the Shuttle Program, NASA promised a launch of every three weeks -- something they knew full well would never happen. While it was reusable (well, the orbiter and the boosters anyway), it really was meant to work with a space station -- that is, Skylab. But it wasn't ready in time, so we sat out of space for years.
Now we have a new station that took way longer to build than we expected, which they want to deorbit soon. Frustrating!
IMO, the US should have run a long-term successor to Apollo from the end of the Apollo Applications Program (read: Skylab + Apollo/Soyuz) with the goal of setting up a permanent base on the moon with an eye on a sucessor for Mars.
But, as when you live in a house for too long (or have a job for too long), you stop being objective and stop planning for the far future. This is how we got where we are today -- a NASA that does somethings brilliantly and others not so much.