TCAS requires a radar and radar equipment is very heavy.
Actually TCAS doesn't require radar to operate does it? As I understand, it uses radio to transmit an aircraft's location, altitude, heading and airspeed and to receive similar information from other TCAS-equipped aircraft. Using this data, an unmanned vehicle could work out the course to take to avoid a collision and one could be more certain that the correct, or consistent, avoiding action would be taken every time without without human intervention. The need for a radar is purely to display to the cockpit crew the location of the conflicting aircraft. It will still need a weather radar, I presume.
That having been said, I agree that the other factors mentioned in previous posts would weigh heavily against an unmanned commercial aircraft.