... Because that's how I believe it should be, considering MK universe has its own pantheon of deities which parallel the deities from real life religions. That said: if Shinnok has his closest "non-fictional" equivalent in Satan (as Raiden, e.g., has in Thor, Zeus, Indra, etc.), wouldn't that give him all the roles that Satan has?

Plus, now that he vanquished Lucifer and claimed his position as the ruler of hell, there is one more reason why he should be able to have control over the energies of wickedness in the Universe, even though he might not be able to spring himself from the Netherealm unless he obtains special prerequisites (eliminating Shao Kahn and reclaiming his amulet).