Without some internal game logic the storyline will implode! We need some stupid reason for his resurrection or we will be angry! (Actually a simple: "It didn't kill him" would be enough for me as Diablo is bad-ass, also "Not even death can save you from me! *demonic laughter*" is a good arch-villain line.)

"That surface that tells the Heavens it hath an end, cannot persuade me I have any."