Actually NASA has said that all the amateur videos were tremendous help in figuring out what happened to Columbia, so I don't think they are landing in the dark for that reason.
Some speculate that the landing path was moved over the Gulf of Mexico to prevent endangering people. Discovery has several other landing windows on Monday in the daylight if they miss the first one.
The landing path is determined by the orbit inclination.
The ISS is on a 50 something degree inclination (for the Russians sake).
If you noticed the launch, the vehicle went way North in order to reach this inclination.
For non-ISS mission, the inclination is in the 30-something degree range