A lot of the time, a Zed may choose to intentionally ult an opponent to dodge an attack, and then quickly recast to switch back whilst blowing his ult cooldown in exchange, now it's kind of pointless since it basically equates to flashing into them instead. This directly effects his outplay potential since he just lost a vital tactic/means of doing so.
Casting speed was his main tool tbh (as most outplay vids on yt will attest to), since every other assassin came with things like hard CC, shields, stealth, innate sustain, resets, or other form of utility/range which he has none of in return for pure offense/shadow play - which was too hard to follow for players so lets nerf the speed/cast times AND throw in delays cuz fk it (as well as nerfing his splitpush so he becomes even less useful past laning). This despite him probably being the assassin with the easiest means of shutting down/countering from Zhonyas/QSS/Exhaust/FaceoftheMountain/Mikaels/Heal/etc. - most of which have seen buffs to affordability and being part of most builds anyway, as well as plenty of picks with the ever more present invulnerability shtick, to plain old tank/ADC-favoring meta (with even AP's given better itemisation), or just having any form of CC assuming you haven't just grouped to siege. He was already weak late, hence at least restoring it to 0s at rank 3 shouldn't be that big a deal. I mean, most of the time they nerf a champion it's to their early game with their damage/stats restored by lvl 16 (or even buffed to emphasise their strengths), why not Zed then? Meh.
It's like what if you put a 0.5s delay on Leblanc's Distortion since she has the advantage of range and being able to cast it more as a regular ability? Watch people go apeshit over that.