IB takes five stacks of Wrath to execute, which means two and a half combos (or two combos and either of Berserk/Vengeance, the second of which is already a damage mitigator). This takes 8 GCDs, so about... 12 seconds. This is less than Rampart's 60 seconds, yes (provided all attacks hit and you don't have to dodge away due to AoE or the boss has Invulnerable/Gone moments). It also doesn't last as long as Rampart (only 6 seconds vs 20), so it's trickier to work with, plus it needs a target to execute.
(Example: Catching three balls in A4. PLD can pop Rampart, and it'll last for all of them. WAR can pop IB by whacking a leg before running over to the balls, and IB will most likely have worn off by the time she gets there, and if not it'll be gone after the first ball.)
Add to that that WAR only has one other damage reduction (Vengeance, as mentioned) that effectively works, since Foresight only works on physical damage. Thrill is kind of damage mitigation, through a self-heal plus increased max HP of the same amount. The new Equilibrium is also a self-heal -- or TP heal, depending on stance.
(And, of course, that a PLD can always Hallow up in case of emergency while chasing said balls and taking Ray to the face... Holmgang isn't quite as useful there :p)