As I've mentioned previously, there's Necromancy and then there's Necromancy.
One covers any magic used to manipulate mortal souls or bodies after death, the other is a school of dhar-based magic invented by Nagash.
The Druchii make use of the former, same as the High Elves (although in a more restrained manner) and many others who use Shyish. However, they would never use the latter, for the simple reason that it was invented by a human, and furthermore elves are uniquely ill-suited to practicing Nagash's teachings.
It is a brand of magic that resonates with the emotion of "I don't want to die. I want to defy death itself.", the same feeling that drove Nagash on the long road to becoming the Great Necromancer. And if there's one thing humans are better at than elves, it's being afraid of death. Certainly, elves are afraid of death, especially of the violent kind, and so they are capable of learning Necromancy if they really try, but it would be a struggle for them to grasp what comes easily, almost instinctually to humans.
Elves simply fundamentally lack the psychological capacity for the kind of scrambling, desperate need to prolong your own life that comes from knowing you have only a scant few decades to live.