Because Zed is super feast or famine, and his feast cases are also very hard to achieve at the moment. Against competent players, Zed will have a VERY hard time getting fed in lane and being successful later in the game. And if he isn't killing things he is completely, utterly useless.
Do I think he is a pain in the ass? Yes. His lane phase is oppressive to the person he is laning against. But with items that hard counter his assassination attempt (hourglass, QSS which just got buffed, hell even death's dance) as well as decent team coordination, Zed will never succeed in the current state of the game.
I'm fine with him returning to a point where he isn't useless, but I do worry about him becoming too strong again. That wasn't fun.