Because the LK can't raise paladins from the dead as more paladins.
Allied race DKs actually bring up something interesting though. We know the machinery of death is broken and that the Jailer is sucking souls directly into the Maw, acting as an intercessor between souls and the Arbiter. So it sounds like Bolvar has figured out that this is happening and is able to either pull souls directly from the Maw or to act as an intercessor himself to pull souls to him before they enter the Shadowlands.
Either way I'd consider it supporting my theory that the LK - regardless of whom it is - is the "usurper" that Sylvannas talks about when she enters Icecrown. That the LK is usurping the powers of the Jailer.