Well part of it is there's only a handful of endgame content, and half of that endgame content is centered around "speed" runs where time completion is of necessity, making WAR's high AoE DPS and surviveability much more desirable since PLD has significantly weaker DPS and no AoE DPS moves. With the other end-game content like the Primals and Dungeon Bosses the D-Lv (difference in level) calculations greatly affect defense's usefulness making WAR's HP a bigger asset in some cases, throw on their good DPS and it still makes WAR preferred in general.

The other part is a stigmatisim, PLD has been on the backburner to WAR for such a long time it's going to be hard to break that mentality despite the great updates PLD has been getting making it into a somewhat competeable tank. Which I'll bite it still needs some work, but it's getting there. However, beyond that point it will be up to SE to make content more friendly to single-target jobs like PLD, MNK, and DRG.

It's really up to you to decide weather or not you'd like to play on PLD, nobody will fault you for it, but don't expect thing's to be all peachy either you might end up fighting an uphill battle when it comes to the mentality of the playerbase.