For anyone looking for a quick summary, the problem is orbital inclination (i.e. how tilted the orbit is relative to the equator). Columbia was in a 39 degree inclination, and for reasons related to physics (their launch site is at a high latitude) and politics (China lies to the east) the Russians can't launch into inclinations less than 51 degrees. It's possible to change inclinations after launch, but requires a metric buttload (technical term) of fuel.

This is all leaving aside the fact that the Russians (or, indeed, anybody) don't just have rockets sitting around ready to go at a moment's notice.