The thought that it was perhaps a one-off vessel put together by an isolated Shivan faction certainly has some appeal. For them, there are no reserves, no massive juggernaut fleet to fall back on. They have little choice but to put all their eggs in the few Demons they have, and perhaps they lost one, or at least took heavy damage to one from the Ancients(pure speculation). They, perhaps lacking the resources and facilities to create something with the size and hull integrity of a Sathanas, decide to build the closest thing they can to their own sort of 'Colossus', mounting multiple destroyer-class reactors, driving a nigh-invulnerable shield system and massive beams.
Perhaps, as others have suggested, the primary(FS2) Shivan fleet doesn't care that much about the loss of individuals or even standard warship crews all that much, but a fully separate group of Shivans, all by themselves, has a reason to place the survival of their command(or whoever else was in the Lucifer), as a real priority.