Why might some former cuckoo host species lose their egg-rejection defenses?

Former host species may lose their egg-rejection defenses because the evolutionary pressure to maintain them is relaxed once they are no longer targeted by brood parasites like the cuckoo.12

In the context of coevolutionary alternation, cuckoos shift between different host species over many generations. When a host species develops effective defenses—such as the ability to discriminate and eject foreign eggs—cuckoos may abandon that host in favor of others. Once the threat of parasitism is removed, the high cost or lack of necessity for maintaining these complex defensive behaviors means that, over many generations, the evolutionary pressure to retain them diminishes. Consequently, these former hosts may lose their defenses, which can eventually make them vulnerable to being targeted by cuckoos once again.12