[Question_Assumptions]

From what I understand, the Columbia was using one of the older style external tanks that was a bit heavier, which was part of why it was in a lower orbit and couldn't reach the space station. They figured that a lower orbit science mission would be a good use of a tank they couldn't use to service the space station.

[tricky_k_1972]

It wasn't just the tank, the shuttle itself was heavier than the others, this type of science and repair of the hubble were about all it was up to (no pun intended).