Easy,
No aircraft "has" to use max reverse thrust on landing. Sounds like it was pilot technique. Certainly, pilots will use more reverse on a short and/or wet runway.
Reverse thrust will contribute to the stopping distance based on the retarding force. Assuming a constant reverse thrust force (not necessarily the case) then the reversers will dissipate more of the aircraft's energy if used at higher speeds, versus used at lower speeds. Hence, a 10 second use of reverse at higher speeds will mean the brakes have to absorb less enery than a 10 second use of reverse at lower speeds.
I suspect this is why it is often said "reversers are more effective at higher speeds"
Hawk