It really isn't, lol.

HP and -dmg are completely separate stats and perform completely separate roles. From a spike standpoint, sure, they are similar in their operation.

From a realistic standpoint, HP is purely and simply burst prevention. To run X dungeon/raid/primal, the tank will need Y HP. Any HP beyond this point is wasted.

-dmg taken is a stat that effects the tank over time, and by its nature the healers MP pools, the bards DPS, etc. In addition to this Paladin has the other list of tools to soften the blow of any large, undodgeable attack and they're all fire and forget.

The discussion isn't that Defiance HP buff and Shield Oaths -dmg buff don't function as the same thing when hit by a big ability, but that they don't function as the same thing in a sustained engagement (which they do not). On top of that, the Paladin is more sustainable when left to themselves in the case of emergencies whereas a Warrior is most definitely left in the wind.