It was just easier to the STA for Challenger and structural spares for Endeavour because it only involved construction. Use of Enterprise would require a lot of tear down and gutting before it could be rebuilt.
I thought I remembered reading that Enterprise was something like 0.5 deg out of alignment, too?
While on the subject would it have been cheaper to refit enterprise or build endeavor after the challenger incident?Or was the refit issues just too costly and unsafe for enterprise to ever be a spacecraft? (Yes I know enterprise was property of the smithsonian at the time so ignore the politics)