Why Nelson's third turret was not superfiring

Why did the Nelson class place its third main turret low rather than superfiring over the other two?

The lower position allowed the three turrets to be packed more closely, keeping the ship shorter and lighter than a raised third turret would permit.

A superfiring third turret needed additional longitudinal clearance and changed the relationship between the guns and conning tower. The historical arrangement compressed the armored and machinery-related length of the battleship. That saved weight where treaty displacement made every ton important. The awkward firing arcs were accepted as part of a broader effort to concentrate the main battery economically.