C. Hammond Query ("Basics of Crystallography and Diffraction") -- Hard Sphere Model

I am currently studying C Hammond's "Basics of Crystallography and Diffraction" (third ed.). In the first chapter ,concerning hard sphere model, I have found the following statement:
There are, in fact, no examples of elements with this structure because, as the model building shows,the atoms in the second layer tend to slip into the ‘hollows’ or interstices between the atoms in the layer below. This also accords with energy considerations: unless electron orbital considerations predominate,layers of atoms stacked in this ‘close packed’ way generally have the lowest (free) energy and are therefore most stable.

I cannot make sense of what he meant by "unless electron orbital considerations predominate" and I'd be thankful if someone could provide me with an explanation.