Zed is not as hard to beat in a full game as LeBlanc or Ahri.

Zed is oppressive in lane, but lane is just part of the game. Don't feed him in lane, and he falls off. Meanwhile, Ahri and LeBlanc don't really fall off and are oppressive all game long.

First reason: he is AD, and they are AP.

AP champs have huge advantages:

1.) not countered by armor which is always a must-buy on both tanks and AP mids (Zhonyas). 2.) many champs don't have level scaling MR but all have scaling armor 3.) access to Zhonya's 4.) much easier access to CDR 5.) usually much higher utility

Second reason: he has no CC to stop enemies from running. Ahri has hard CC and LeBlanc has a slow+snare.

Third reason: they have way more burst mobility than he does with much fewer conditions. Zed has to return to a marked shadow on a long CD. LeBlanc gets dashes on short CDs that can also be returned to along with a Shaco ult passive, making her way slipperier, as well as access to Zhonya's. Ahri has a speed boost on her Q, true damage and 3 burst dashes.

Fourth reason: he is melee with a significant part of his DPS coming from autoattacks. That means if he fails to finish off the ADC, he must either shadow back to his team or risk being kited. Ahri/LB have no such limitations since their burst is all ranged.

Fifth reason: he has delayed burst. That means that while death mark is active, even if it would kill the enemy, they still have a chance to be healed, Zhonya's, QSS, etc. Ahri and LeBlanc don't give you a chance to react.