This is just me, but when I used to ban Zed it was when he was really good at one thing: murdering our ADC which, in Bronze, means basically instant loss if the enemy got him no matter how good of a player I was unless I kept on top of his roams. He's very binary in his methods and is good at that single thing. Sure, when he's ahead he can murder squishies left and right, but his early game is trash against any counter-burst champion like LeBlanc, Annie, or Ahri which then becomes a pure skill matchup.
However, I think what annoyed me with him was the constant Zed mains who just spam laugh and taunt until I shoved their face in the dirt. Because he got a Championship skin, I'm being unfun and he's going to be on my permaban list. Don't care if he's on my team or if the enemy is thinking of picking, be a jerk elsewhere.
Also, I'm not saying every Zed main is like this, probably about 80% or so.