Not enough room for a necro with Unholy DKs already present. Either they would be highly redundant or the Necro would have to steal a bunch of flavor and spells like DHs did from Demo locks.
They talked about the class issue in 07 when they introduced DKs. At that Blizzcon they were asked about whether DKs would intrude on the roles of rogues esp and to a degree warriors. At the time, they feel DK was distinct enough that it wasn't a big issue but then elaborated that adding classes does, at some point, run into the problem we'd have with necromancers - you start splitting hairs in order to differentiate the new class from what's in the game already.
"It's like a warlock but different but the spells are a different color." Or, "It's not like a DK because they wear cloth and summon zombies instead of ghouls..."
Say it with me now...
Death. knights. Are. Necromancers.
You can put your DK in a dress, transmog them a scythe and roll unholy if you want to be a necromancer.
“Do not lose time on daily trivialities. Do not dwell on petty detail. For all of these things melt away and drift apart within the obscure traffic of time. Live well and live broadly. You are alive and living now. Now is the envy of all of the dead.” ~ Emily3, World of Tomorrow
Words to live by.