You may want to look up some of the shuttle history. Carrying out experiments in space was not the original idea. That was what the space station was for.
The original concept was a smaller vehicle, intended to move people and small cargo back and forth between a permanent manned space station. It was truly intended as a *shuttle*. It was intended for frequent launches; hence the interest in a reusable vehicle. Heavier payloads were intended for conventional rocket designs (some kind of Saturn evolution).
But then funding was cut. Getting a new heavy lift booster, a space station, *and* a shuttle was not going to happen.
At the same time, the Air Force got involved. The AF needs the ability to launch spy satellites in to polar orbits. By working together, the thought was that STS could be kept alive. But polar orbits are harder to reach, and spy satellites are big and heavy. That meant a much larger vehicle. So the shuttle design evolved into what it is today.
But then the Air Force realized that the compromise design was lousy, and decided to stick with conventional rockets. SLC-6 was never used.
As a result, NASA was stuck with something of a white elephant. The shuttle was trying to be too many things at once. It wasn't the small, cheap "bus" that was originally conceived, but it also wasn't a cost-effective heavy launcher.
It's a shame; some really brilliant technology and engineering went into the program. But when the design goals are conflicting and ever-changing, no amount of engineering skill can compensate.