Supply and demand.
Do you not recognize that fans legitimately ask for a Necromancer class?
Again, we're not talking about whether it'd likely happen or whether there's even a sensible place for Necromancers. We're talking about what makes sense, and the main purpose of a Necromancer class is to fulfill a Necromancer class fantasy which players are asking for. Poisons and Necromancy is all within that theme that fans recognize. Death Knights and Warlocks have never fulfilled or replaced that demand, and this needs to be acknowledged considering the same niche arguments were used against Demon Hunters and we know clearly that being a niche concept does not get in the way of them becoming a new playable class.
The argument I present makes perfect sense. Where is your confusion?