I think there are multiple pieces to this puzzle. Thrust reversers redirect thrust forward, but that is not their only effect.
On the old stovepipe turbojets, clam-shell thrust reversers closed off the entire back of the engine nozzle. They not only redirected the engine exhaust forward, but the exhaust was also pushing backwards on the clamshell as it was redirected, thus pushing backwards on the whole airplane (by way of the nuts and bolts that held the clamshell to the airframe). In addition, the entire normal thrust flow was blocked, not just the fan bypass thrust. Finally, the clamshells stuck out into the slipstream, acting to a small degree as speedbrakes, not unlike the tail speedbrakes of a Bae 146.
That last effect is very speed-dependent, since drag varies as the square of the velocity. Slow by 50% and that drag drops to 25%.
On high-bypass fan reversers:
1) the engine core is still producing forward thrust, even when the reversers are deployed. Some tail-engined planes have full buckets that block core thrust, but many have a slot in the center, and mostly reverse only the bypass air. So the engine is pushing in both directions at once.
2) Some fan reversers of the "slide-back" type (Boeing, mostly) do not stick out into the slipstream, but others still stick out to the side (Airbus, e.g.) and will produce "speedbrake" drag as well as redirecting the thrust. But exponentially less drag as speed drops.
3) Even on the "internal" Boeing-style reversers, doors drop into the bypass flow, and are being pushed backwards directly by, and blocking, the fan thrust.
Combine those with the supposition (in the thread linked by DaveReidUK) that the reduction of ram air pressure into the engine intake as the aircraft slows reduces the thrust available to "reverse", and you can see how they add up to reduce reverser effectiveness with decreasing speed.
Less door drag - less internal backwards pressure on the reversing plates/doors/buckets - less total airflow - increasing (percentage-wise) interference with slowing from the remaining forward core thrust (at high throttle).
An aircraft can be stopped with reversers alone - but below a certain speed, it takes longer than shutting them down and going to wheel brakes, and increases the risk of FOD damage.