It could be that Necromancy itself (by Eorzean definition) is the combining of White Magic (life/restoration) and Black Magic (death/destruction), which would explain why it's not common. Each are rare themselves due to being forbidden. Someone adept enough at magic to have the capacity to mix them would result in a unique and frightening type of magic (and I'm not talking about a job like Red Mage that utilizes magic from each individually, but rather a merging of the two).