Actually, there are lots of flightless birds. Penguins for example, mind you they do have wings, but on the whole I would think they had it a bit harder with those stubby little legs. Ostrich, Emu, Rea, Kiwi, Cassowaries, Kakapo (one of the rarest parrots in the world, and also a nocturnal bird), and a bunch of others. Some of these have wings and some do not, so flightless birds are not altogether rare. The main difference here is that the flightless chicken got it's flightless characteristics in one generation as a result of a genetic mutation whereas natural flightless birds evolved that way over many many generations so are more adapted to there lack of flight.
Urban Coyote