Why are the wings of a kiwi bird too small to be of any use in flight?

Kiwi are small flightless birds which live in New Zealand, a land which, for thousands of years, was free of significant predators. Because of this, the kiwi did not need to fly, so it has wings which are too small to be of any use in flight. It finds its food on the ground and nests in burrows it digs (or old burrows it finds) so it has not required wings to escape predators. Unfortunately, the introduction of predators as a result of European setlement has led to the endangerment of several species of the bird.