I think it's because it's seen as a perversion of the natural order, and usually ends up putting the souls of those its done to into torment and pain (I mean not always, but that seems to pop up quite a lot).

I was thinking that the undead kingdom probably weren't actually raised by a necromancer, mind you; more that either the land they originally came from was rich in magic and when they died they all just got back up again, or they were wiped out in such a way that their vengeful souls couldn't rest and so they... Just got back up again and after dealing with whatever wiped them out found themselves unable to pass on anyways.  But hmmm, major work in progress I guess. ^^;