Why are some birds incapable of flight?

Ostriches, emus, cassowaries, rheas, and kiwis can't fly. Unlike most birds, their flat breastbones lack the keel that anchors the strong pectoral muscles required for flight. Their puny wings can't possibly lift their heavy bodies off the ground.

Explanation:

cuz they're fat and their wings are small

Explanation:

their flat breastbones lack the keel that anchors the strong pectoral muscles required for flight

they lost the ability to fly through evolution because flying was no longer needed