It is exactly that they already have a working meta that IMO it is just easier to expand from it. People who want to stick to their tank stance still have the ability to do so, while they can work on their own iteration of roles and changes on skills to suit each job's playstyle.

PLD has no issue with aggro sustain even in SwO, DRK on the other hand has way more issues without having to trade off dps for aggro. In design, DRK is utterly shit with so many conflicting kits. This is also why NIN is such a key piece to bypass DRK's weak aggro for pushing damage in a high skill team setting. What do you think about DRG NIN BRD MCH ruling the current top 4 DPS slot? AST over WHM?