Off-GCDs are extremely strong, so if there's any reason they aren't oGCDs then that's probably why. I do agree that Bloodspiller and Quietus would feel a lot better as oGCDs but it's not likely to happen unless their potency is nerfed a bit or a longer CD is attached.
May or may not be right but I'll take a stab:
Souleater combo with DABS and DASE
Bloodspiller on GCD / 2.42 GCD:
150 + 250 + 440 + 540 = 1380 potency / 9.68 sec
140.81 pot/sec
Bloodspiller off GCD / 2.42 GCD:
150 + 250 + 440 +540 = 1380 potency / 7.26 sec
190.08 pot/sec
So it's a significant increase in DPS by making it off-global. Of course we can argue about DRK needing a bit more DPS still, but just wanted to point out how making it off-global it would need to be carefully adjusted.
