Why don't some birds fly?

Answer 1

1. There's no need to fly.
2. Some bird's wings aren't designed for flying (like penguins)
3. Some of the bird's bones (like in ostrich) are hard not porous and light. So it's impossible for that bird to lift it own weight.

Answer 2

The structures used for flying, and flying itself, take up a lot of energy. In some areas, due to a lack of predators or another way to protect itself, some species of birds have evolved to walk permanently on the ground in order to exploit this niche and devote the energy it would have used in flying to other pursuits.

Answer 3

Presumably because they don't need to. What does flying allow the bird to do? Now think about the non-flying birds; do they have an alternative approach to that factor? Perhaps they don't need to do it; perhaps they can handle it in a different way.