When they decided to normalize all tanks in ShB, they decided Unleash, not Abyssal Drain, the ranged enemy targeted spell, would be the best one to define DRK's unique identity. Unleash, the spell that behaved the same way as Flash, Total Eclipse, and Demon Slice in a world without TP, and eventually Overpower once they decided conal AoEs were bad (except for new melee jobs. Old ones can't have them.)

Not Abyssal Drain, the spell designed to let DRK stay alive on a giant trash pull in a way that would make WAR jealous (and eventually, another thing stolen from DRK).

I dunno, they probably just weren't satisfied with its appearance or something. Even Carve and Spit that it's strangely tied to kinda went from being a multipurpose tool into just another oGCD damage button when they chucked Dark Arts.