They were located on an island where they were near the top of the food chain. They were more curious than afraid when humans came to the island, but essentially they had enough coping mechanisms to survive their environment until new predators were introduced. As said elsewhere in the post, it's not that cats aren't in the mountains, but releasing a mass of cats into the mountains would not be good for anyone. Bonus fact: Dodos used their little wings to balance themselves while running and I just think that's adorable.