The expressions for the field include terms of different powers of r. At large distances (compared to the wavelength) from the transmitting antenna, only the r^-1 terms (which dies off with r more slowly than the others) remain with any appreciable strength. This is called the "far-field". The power per cross-sectional area is proportional to E·B (or, equivalently, to E^2 or to B^2), and therefore goes as r^-2.