They don`t have them. And there`s nobody to put them next to the others
Don`t forget soulstones are a craftworld invention, while Dark Eldar are what`s left of the regular eldar :P they don`t even use wraithbone.
Plus, in a soulstone your soul is trapped there for ever and ever (or until someone breaks it or puts it in some machinery). Whereas in the warp it gets the sweet relife of death :P most likely a painfull death if Slaanesh happens to be around
but a death none the less.