An effective reflector results in minimal signals behind the array. Thus additional reflector elements placed behind the first reflector receive only a very small residual signal going past the reflector element and therefore contribute little to the signal leaving at the front of the array. Occasionally, a multielement reflector structure can be found, but this is ready just a different geometry at the approximate distance of the primary reflector. Sometimes a plane reflector is used behind the driven element for better front-to-back performance.
Additional directors, on the other hand, are in the path of main beam formed from the driven element, any previous directors. Thus they continue to focus the beam further as it progresses forward along the antenna axis.