Why were Nelson and Rodney unusually hard to disable?

Why were Nelson and Rodney more endurable than many other battleships?

They used compact machinery, low speed, and an all-or-nothing layout to concentrate an unusually large share of displacement into thick protection around a short vital core.

Most battleships spend substantial displacement on the machinery needed for higher speed and then spread protection over a longer hull. Nelson and Rodney used highly weight-efficient engines to obtain only the speed required for the battle line. Their all-or-nothing scheme then concentrated armour around a compact block of magazines, machinery, and command functions instead of thinning it across the full ship. Water carried in the underwater-protection system added further defensive mass outside standard displacement. The result did not make either ship invulnerable, but it meant an enemy needed sustained damage or a fortunate hit to stop one decisively.