How is there no detailed backstory for Undead holy priests? How can there be when there are, say 1 in a million undead priests is holy. No Undead... Well, no self-serving Undead would choose to channel the Light, when they can channel Shadow instead. Besides that, Undead Priests belong to the Sect of Shadow (Something like that).
There is a backstory, it's just small because lore-wise, there at few, if any, Undead holy priests.
EDIT: Blizzard did explain Undead holy priests. If you don't like their explanation, too bad, it makes sense to everyone else.