Simply put, a WAR needs to pop Inner Beast and a defensive CD for the big hits while a PLD only needs to pop a CD. If a WAR goes into a fight with only a vague understanding of the timing of those big hits, then they will not be able to mitigate properly. PLD can mitigate everything on reaction as long as they are frugal with their CD usage.

You also tend to get a lot of WARs that spec STR and use (old i110) STR accessories without having the overgeared left side or healers to back it up. Random PLDs almost universally go full VIT.