Why Nelson did not stack all three turrets

Why did the Nelson class not use another superfiring turret?

Because lifting another roughly 1,600-ton turret would impose major displacement and metacentric-height penalties.

The proposed comparison is with ships that stacked more gun positions vertically. Such an arrangement would have been possible in principle, but it was not free. Nelson's main turrets weighed roughly 1,600 tons apiece. Raising another one would add topweight and affect the ship's metacentric height as well as its total displacement. The design problem was whether the hull could carry the resulting stability and weight penalties.