Why is the kakapo flightless?

The poor thing is a very heavy parrot, its wings are too small for its size/weight and it hasn't got the pronounced keel bone that anchors the flight muscles of flying birds. (Not all birds can fly, but that is another matter!) The Kakapo also accumulates a lot more body fat, proportionately, than any other parrot. (See Related links below)

However, this bird always wins the heaviest parrot contest!