Why Nelson’s three turrets were not all superfiring

How bad would it have been to build the Nelsons with all three main turrets superfiring?

It would create stability and firing-force problems for little benefit, since the guns could already fire forward at normal ranges.

Stacking all three Nelson turrets in a superfiring arrangement would put more weight high and forward. The ship would also have to absorb the firing forces of the elevated third mounting in an already demanding part of the hull. At normal capital-ship ranges, the guns' elevation allowed the rear turret to fire over those ahead without physically raising the entire mounting. At closer ranges, a small course change could clear the arcs, so the stability penalty would buy little practical firepower.