Flight requires extreme adaptations that make terrestrial life much more difficult. It’s hard to evolve it and once you do, going back will be difficult. Especially when competing with a bunch of animals that haven’t completely warped their bodies for a totally different lifestyle.

On isolated islands without mammals, many birds do begin to revert to a terrestrial strategy. But the mammal body plan is just better for this, and when mammals have reached such places, flightless birds have almost immediately experienced population collapse.