A lot of the effect of thrust reverser performance is related to inlet performance. At slow air speeds, the air entering the inlet must accelerate. This acceleration inside the inlet results in the inlet being pulled forward. Another of way of explaining it is that at slow airspeeds, the inlet produces a significant amount of thrust. As airspeed increases, the air entering the inlet no longer accelerates and in fact begins to decelerate and create negative thrust (which is accounted for as "ram drag"). So when the aircraft is at flying speed, the inlet is producing no forward thrust and, with thrust reversers deployed, the fan thrust is negative (the core thrust remains positive). As the aircraft slows down and the engine RPM remain high, the air in the inlet must accelerate more and more, producing more and more forward thrust in the inlet, effectively reducing the net reverse thrust.
There are two other reasons airliners don't use reverse thrust at low speed.
1. FOD. The thrust reversers not only direct airflow forward, but also radially outward. Thus the bottom section of the engine nacelle directs airflow downward toward the runway/taxiway, potentially throwing up debris into the engine.
2. Re-ingestion. As the airspeed goes down the reverser output (efflux) angle moves forward. Eventually, the reverser efflux will re-enter the engine inlet. This causes all sorts of problems for the compressor and will often result in serious compressor stalls which can seriously damage an engine.
BTW, the C-17 is unique in that it has both fan and core reversers. Further, the lower section of the reversers are blocked off, resulting in all the reverser efflux moving forward and upward. Very little moves downward. Further, the flow is carefully tailored so that the reverser efflux does not enter the engine even with negative airspeed (i.e. a tailwind), meaning that the reversers can be used to back the aircraft using engine power alone.