We discussed Columbia's orbital inclination on this thread back on 1/22.

Columbia wasn't in an orbit compatible with the ISS (it takes more than just being in the same inclination, you also have to be in the same orbital plane) because the mission didn't call for it. The cargo bay didn't include an airlock for docking with the ISS because it was carrying the SpaceHab module. They were doing the experiments in the SpaceHab instead of the ISS because the ISS labs aren't fully operational yet and because the SpaceHab was available to configure for short term experiements.

Given that Columbia was the heaviest of the Shuttles, it's possible it might not have been able to carry the SpaceHab into an orbit compatible with the ISS. The higher inclination reduces payload capacity to orbit.