The long as short of it is because DRK didn't exist in 2.0.

Back then it was only PLD and WAR, and we only went up to 50. PLD was the damage mitigation tank, and WAR was the life steal one. So when they added DRK in, they couldn't really make it focused on drains, since that was still WAR's thing, and made it more parry/dodge focused at release. And from there it was mostly just the jobs evolving in different ways, with PLD gradually getting more healing abilities as it went on to align it with legacy expectations, and DRK just.... kind of getting whatever the other two weren't and changing constantly and then just sort of ended up here since it's not like they have any real direction with the job anymore.