WAR has amazing defense. It's just, outside of damage and defense, they bring nothing to a group. DRK brings a ~5k HP shield it can give other tanks and has good magical damage defense, which is important in most of the new raiding content. It can be argued which is in a better place right now, with most choosing DRK just because of how much easier it is to play.

That being said, PLD >>>>>>>>>>>>> WAR/DRK, so if you want to be the best tank of the moment, PLD is your pick. The reason people don't all go PLD is because groups generally only take 1, so the other spot has to be filled with something.