Why were flightless rails on small islands especially vulnerable to extinction?

Flightless rails on small islands were especially vulnerable to extinction primarily due to their lack of defensive adaptations against predators and the arrival of humans and introduced animals.12

Many of these birds evolved in isolated environments that lacked native mammalian predators, leading to a condition of ecological naivete where they did not develop the necessary behaviors to escape or defend against new threats. When humans arrived on these islands, they often hunted these birds, which were frequently described as unwary. Furthermore, humans introduced nonnative species—such as rats, pigs, dogs, and domestic cats—which preyed upon the birds, their eggs, and their young, or destroyed their forest habitats. Because these flightless species could not use flight to escape, they were unable to survive the sudden introduction of these predators and the resulting environmental changes.32451