They gut certain champions because they show exceptional strength in high levels of play. It also revolves around how impactful that one champion is with relative to gold acquired in a time span and in what ways. Sure, Zed can annihilate one player with relative ease. That's what his kit is revolved around, and the impact that has on a game isn't quite as huge as someone like Azir who shreds anything and anyone off of two items, or Kindred who is the most dominant jungler for a time now.
His win rate is below 50%. Partially attributed to his high ban rate, but his kit is not something Riot is willing to overnef because it does not need it. Zed was designed as a counter-skillshot mage champion with all of his mobility and single target damage. He will forever remain that way. People now ban him because it's "what you're supposed to do." Many ignore the obvious counterplay of exhaust, correct CC timing, and zhonyas.
Compare Veigar and Zed. Both have kits designed to annihilate someone. However, their powers are distributed differently. While Zed is given a selfish kit designed to keep him and only him alive and well, Veigar's also brings team wide utility in many forms such as extreme zoning, CC, and more aoe damage. However, with both having the same sort of kit objective, being break one player in half without them having much of a say, which one is more likely to be heavily nerfed? The one that is designed to hit one player and get out, and then be heavily hurt in play making potential until CD is up on ult, or the one who can delete someone with less effort, and still provide extreme threat and utility.
i'm not calling you personally out on this, but many if not most look at Zed's strengths and what he does and overlook the weaknesses. I admit, it's infuriating to be popped by a Zed. His kit is mentally toxic, in a "cool guys don't look at explosions" sort of way as well as his laugh. Also people have some ridiculous notion that they ought to be invulnerable under their tower, which feeds into the Zed hate. Half health under your turret doesn't mean Zed shouldn't be able to kill you. You're under your turret, the advantage is still yours. And if you have exhaust, flash, or zhonyas, you are perfectly safe. However some mages need to live and die by rushing mana regen, which is their undoing. The argument that Zed can endlessly spam is currently being addressed by making his "spam" do much less damage.
They are transfering his power in a nerf-like manner. The damage he loses is being made up in a different way, but the way to unlock that power is far harder. Therefore, you will lose damage in a match. To unlock his transfered power, you now need to blow his long, mobility CC or even worse, his ultimate. Instead of hitting one button, you now need two. The second is always a long CD.
People have always complained about Zed in the laning phase. People accept that he falls off late game a bit and that he has healthy counterplay throughout the match. No one complains about zhonyas, they just complain about rushing it. This is Riot's way of making the place where he is problematic, more even. A flat nerf simply nerfs the early game and the late game.
They also love Zed's kit. What's not to love? They find Zed's kit the definition of a healthy assassin. High, single target damage, get in and out, mobility on punishing CDs. And Zed's kit was balanced for the longest time. It was items that unbalanced him. Youmuu' namely, being a high damage, 20 armor pen rush item with a great active and hexdrinker being so good against nerfed-mana regen mages, along with armor pen on maw. This is why they don't want to flat nerf Zed's kit, because the kit is not the problem. Zed remains to be one of the highest skill cap champions, and this transfer of power reinforces that. Basic Zed's lose .1 AD ratio and base damage, those that can actually use Zed's shadows and intended skill cap will see increases in performance in KEY moments. It's not as if the Zed mains will consistently put out higher numbers. They still will have times they can't afford to burn the shadow or where they just can't and will have no choice but to use regular Q's. However, it opens up higher potential in key, small windows of time.
Kassadin's ban rate was upwards to I believe 97-98%. He was banned because of unparalleled mobility coupled with extreme burst damage. He had aspects of his kit that prevented counterplay from any champion in the game, being constant mobility, long silences, and slows. Along with very good AOE damage, played properly. Zed does not come close to these qualifications.
And lastly, an unfortunate aspect to balance but a relevant point nonetheless, Zed isn't in pro play. Pros know the game best. When Riot sees the LCS/LCK and MSI run rampant by Azir, Ekko, Poppy, Kindred, Graves, Lucian, Ezreal, Maokai, Soraka, Braum, they are going to pay attention to those. And honestly, for good reason. Zed was only banned a time or two and only picked a couple times. He did not see any spectacular success either. Champions like Azir getting their ults absolutely slashed stem from MSI where Azir was essentially pick or ban, and when picked, the ult was shown in performance to millions watching what it did. This gets people talking about balance, and Riot needs to look into it.
My personal opinion, Zed is overrated. I am a Zed main, I admit. Climbed to gold S4 and S5 with Zed. Many wave me away and call me bias. Maybe. But personally, I know more about Zed than the average non-main. I play Zed, I see what he can and can't do. The laning phase is easy. CS is easy. Early game my spam wins. And Riot is nerfing it. They nerf'd W's CD so less spam and mobility, they are nerfing Q's direct damage so less spam damage. They are hitting him many times. But nothing ridiculous like lowering each rank of ult's damage by 10% or something. Zed has counterplay, and people that are keen on it and utilize it perform and excel versus him. Proper wards, pings, and shot calls can easily thwart roams, which is what he is famous for. Zed, unfortunately, is a low ELO Solo Queue stomp. Where wards are few, pings are a luxury, and the lanes play for kills and not CS, Zed has it nice. But, Riot does not want to nerf their favorite assassin design based on poor players.