Zilean Mid was only a no-counterplay lane for assassins, and only against Zed/Fizz who relied on their mobility to nullify skillshots.
Zlean mid went OoM extremely easy. Push him and he needs to use mana to push or loses tower, outsustain him and he goes OoM in trying to harass you quickly, outrange him and you can counter harass him when he needs to get closer. Play a goddamn defensive lane he falls off like a brick. He also needs 2 mana items to function properly while those he was there to counter can jump straight into raw damage.
Zilean mid was merely a counterpick, just that a counterpick to Riot's favorite boys. So he had to go, cant have the Zilean screwing a Zed's flashy ''muh playz mechanics''.
And despite what Riot tries to do to phase out counterpick and the importance of champion select, the game is build upon it, and continues to be build upon it.
They just decide to ignore that fact when they feel like they need to justificate their nerfs to their problematic favs' counterpicks. And remember counterpicking to use it as a reason to say ''this assassin's kit isnt overloaded/toxic/OP because counterpick!''.
Riot can try to turn around or correct a champion's intended lane like Soraka or Alistar, and that is OK. But both Zilean and Karma were designed with Midlane capability in play. They just got nerfed for being able to play their supposed strenghs, namely, anti-assassin laning. Taric als has suffered from this, as he is also a fighter but top/jg is forbidden for him apparently.