We have three Tanks.
Square Enix want every Tank to be able to Main Tank the Content but that can only work, if all Tanks does close to the same Damage, too.
You can no design a Tank around Mitigation and give him less DMG, if the Mitigation of the other Tanks is enough to tank every Encounter in the Game. Because then, the more Mitigation of the Paladin becomes useless and in a Raid Enviorment around massive DPS checks, Tanks with the most DPS becomes mandatory, which was the result in WAR/DRK Tanks in Alex Savage and leaving out the PLD if possible.