Because all orbits are ellipses which have the primary body at one focus, and the satellite moving on the elliptical path. This means that the center of the Earth must be at the focus of any satellite orbit around the Earth in which the satellite is not constantly under thrust. Orbits at constant latitude such as a 'polar' orbit where the satellite cruises in a circle about the North Pole at a fixed distance from it, are physically impossible because the path is circular, and center of this path does not include the center of the Earth.