Why is a hummingbird the only bird that can fly backwards?

Hummingbirds are the only birds that can fly backwards, sideways and hover in place. They are able to do this by rotating their winges in a circle. To hover they rotate their wings in a figue eight patern much like a swimme treading water.
It is able to rotate its wings in a figure of eight movement that allows it to fly forwards and backwards and to hover.