I believe the controller was getting morning duties completed. No requirement to monitor once departure clearance was given, if there were no other ground aircraft movements or landing traffic. Comair lined up with the wrong runway, which was also unlit at the time, and commenced takeoff.
It’s possible the tower controller was visually managing traffic at the time. Many radars in ATC towers are for reference only, not necessarily for vectoring purposes, and a tower controller’s primary duties are usually visual. Now, this will remain to be seen with the accident investigation what the exact details are.