Why are oxygen requirements based on pressure altitude?
The air in the lungs is in pressure equilibrium with the cabin, but not temperature equilibrium. It's always at body temperature. So the only variable that can affect how much oxygen is available to the body is the pressure of the outside air - thus pressure altitude, not density altitude, is physiologically relevant.

That said, if the density altitude is high it may mean you are hot and dehydrated, and the deleterious effects of those factors may be additive with those of hypoxia.