Well much of the theme of the story is based upon opposing necromancy, and you are Meridia's champion, and she has absolutely zero tolerance for such. They would have to rewrite a lot of the major quests to allow for necromancy, and there is NO way Meridia would choose a necromancer as her champion. Multiple other quests are also based upon the idea of necromancy being vile (which it is,) so this could only work with a TON of scrapping and rewriting content.
They said we might be able play as necromancers in the future? When did they say that? I don't remember this at all.
Necromancy is outlawed in many places in Tamriel, and frowned upon in many more. It would be a tricky thing to add because they would need a reason for npcs to be okay with your character practicing it.
I know this post is more than 2 years old but now that the Necromancer is officially coming June 4 I wanted to say:
FINALLY WE KNOW HOW LONG WE HAVE TO WAIT!