They were not in the same orbit and your fuel only allows minor corrections.
Fuel only allows minor corrections if you are planning a re-entry burn, because you would have to reserve enough fuel for that. If a visual inspection had revealed that re-entry would result in certain catastrophic failure, then at that point a re-entry burn becomes moot. At that point, the question becomes how to save the crew, and at least leave open the possibilty of the shuttle being salavaged at some future time. I don't know if the fuel supply on board for the re-entry burn would have been sufficient to boost them into an interecept orbit with the ISS or not. I do know just enough about orbital mechanics to know that it would take a sequence of several small, precisely timed burns rather than one big one.