Consider a point charge, a spherical shell around it at one radius, and another shell around it at a much larger radius. The flux density at the surfaces varies as the inverse of the square of the two radii. However, the area of the shells varies as the square of the radii, so that the total flux for both shells is the same. Thus, we have conservation of flux going into the space between them and coming out again. If, however, the elecrtic field varied inversely as the cube of the radii, the amount of flux from the outer shell would total less than that through the inner shell, so that the divergence of the field (in places where charge is not) could no longer be zero, and flux would not be conserved. By the way, this sort of situation somewhat resembled that of the strong force inside atomic nuclei.