I heard a NASA rep explain the reasons they couldn't. I believe the crux of the rescue issue was that, unlike castaways on an island, rationing in space is not as easy. There's no effective means (on the shuttle I guess) to maintain adequate air supply for too long. If it were just a matter of food and water, yes, they could string it out.
In addition, they would be running the risk of compounding the problem with another accident. Remember, the incident during launch had not been even close to resolution. They would be scrambling to launch another shuttle without necessarily knowing what happened to Columbia. High risk.
That is, make miserable excuses. That we spend tens of billions on NASA, and they don't have a contingency plan for a Shuttle unable to re-entry is mind-boggling and inexcusable. Thats what you'd expect from a friggin' FRENCH program, not the U.S.!!