PLD is a great tank. WAR is just broken right now it's so OP in how much DPS it can produce as a tank. That's why people are flocking to it. PLD is very much in line with DRK, with the primary difference being PLD handling physical damage better and DRK handling magic better.

That being said, DRK is favored due to Alex Savage relying on physical autoattacks while tank busters are magical. SE already acknowledged this oversight that is leaving PLD on the sidelines and said they will correct it for 3.2 (bosses will have a mix of both physical and magical tank busters and DPS checks will be eased in favor of mechanics checks instead so certain tanks and healers aren't mandatory).