For the same reason as in an Airbus. The reversers help in the deceleration once the aircraft starts braking: they help in achieving the selected deceleration-rate by the autobrake. Once the selected deceleration-rate is attained the reversers have no further effect on deceleration.
In other words: it takes time for the autobrakes to deliver the required brakepressure and achieve the decel rate, up to the point where the selected deceleration is attained the reversers have a cumulative effect.