Mortifer during the quest literally explains why they don't work and tells you that he'll think of how to work on an equivalent specifically for necromancy. So it's a little more than speculation - there is an in-game hint about the future reward space.
Malignius Mortifer:
None of your conveniences for magic will help for necromancy incantations either - say, anything you have to store runes, save them, decrease costs...that sort of thing.
Malignius Mortifer:
Hmm, though we might be able to make some equivalents specifically for necromancy. Hmm, leave that with me, I'll let you know if I think of anything.