The Ostrich is the largest flightless bird in the world and also the largest of all living birds.
The primary reason why an ostrich can’t fly actually has to do with their feathers as such. The individual barbules are not inter-locked like a flying bird’s feather. Ostriches have soft plumage instead of stiff feathers, which protects them from the cold nights and winters that they experience in their natural environments.
Another reason is their skeleton and muscles. 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.
Ostriches don’t use their wings to fly, but they have found many other uses for them. The main use for their wings is for displays of courtship during mating season and using them for balance while running. In fact, the animal is known for its rapid quickness.
Why ostriches cannot fly when other birds can? Because their ancestors had to adapt to their environments, which allowed the evolution of feathers and wings and for some birds to take to the sky and while others stayed behind on the ground.
Source: safariostrich.co.za, blogs.bu.edu, nationalgeographic.com, faculty.washington.edu