Because she was changed from a kinda wonky poison mage into a mana using adc. She went thematically from a slow killing temptress into "lol EEEEEEEE". Because her entire kit was changed from decent damaging DoT's with an ok E into literally everything being useless except for enabling E. Sure, her kit is now 'balanced' and competitively viable, but she's also nowhere near a poison mage and plays like a cruel mockery for the players who actually like poison.
So yes, some people like new Cass. Those people could just play Azir or an actual adc, instead of having Riot murder a unique champion for an AP adc. Riot could make a new poison mage, but that would likely piss fans of old Cass of even more; after all, they just made a unique poison mage (meaning all of the reasons given for not having Cass be DoT focused are 100% bull instead of 75%). If they did it, why couldn't they do it on the actual poison snake lady instead of ruining her theme?