Arguably the entire concept was flawed, fueled more by a sci-fi concept of "wouldn't it be cool if" rather than a sober cost-benefit analysis. It was a space station, built into a rocket, built into an airplane that is also a reentry vehicle - most of the motivation to reuse the vehicle in the first place goes away if you don't make it so damn complex!
It just makes so much more sense to launch big payloads on heavy lift rockets, humans separately on human rated craft, and leave the space station part (robot arm etc) in orbit.
(The one type of mission you can't do without something like an orbiter is satellite retrieval. This capability was used a total of 4 times. Not really worth it.)