The reason WAR sucks as tank is because development has left them there.

First there were NINs starting to tank and blink tanking was all the rage because it was the safest way to go. PLDs got butthurt and cried. SE buffed the crap out of them consistently and now they are decent, but still go /NIN half the time and use other enmity generating moves to keep hate while being able to not only take hits, heal themselves, etc., but also blink away a good portion of damage.

The problem with WARs now (and for a while) is that when NIN showed up on the scene, SE went with it. They ramped up the difficulty on newer "endgame" content. It began to almost require blink tanking. They tried to balance it with a few things that can't be blinked very well (Jailer of Faith comes to mind), but they still really need uber blood tanks... and there's no better pure blood tank than PLD.

WAR doesn't have the evasion to get the most out of /NIN, doesn't have the healing or Def of a PLD, and unlike PLD (and NIN for the most part) can put out big numbers in damage. So WAR is just better at damage because of the way the game got designed after NINs showed up on the tanking scene and PLDs got all buff.

MNK suffers in much the same way. They have even worse Def even though they have a ton of HP. FFXI just doesn't do well with blood tanks in general and SE doesn't even pretend to try to make WAR a viable tank now considering how incredibly better PLD is at generating and holding hate on top of everything else.