Because Riot, once again, were dumbasses.
Upon release, she was a lot more difficult to learn and play, but in return, far more rewarding.
She was a highly specialist mage that took massive amounts of skill and practice and knowledge to play properly.
People who barely played her, or didn't play her at all, whined that she was hard because they were idiots who didn't want to learn how to play her properly.
People who did play her since release or otherwise played her a shitton, said she was perfectly fine once you learned her, which was the truth.
But Riot pandered to the former, made her a generalist champion, let her do too many things, and then gutted every single part of her kit, ESPECIALLY her iconic E damage into complete fucking trash as a result, because they had to because they decided to make her a generalist champion for no good reason.
They changed her unique W casting method to vector casting, which FORCEFULLY ADDED a MASSIVELY LONG inherent human delay on it as well as removed the ability to knock up simply vertically instead of in a direction, as well as made it far easier to learn (VCing is braindead easy).
They removed the lockout on the W (which prevented other spells from being used) which had prevented W-E which was far harder to react to and completely braindead easy. This added a Burst Mage playstyle to Taliyah's kit instead of simply being a zoning pressure mage like she used to be. Now, instead of actually having to become good at Taliyah to land her E-W, people could just do W-E which was absurdly easy. So they had to gut her E damage into utter trash.
Around the same time, Riot added bonus damage on her Q for no reason (to minions). This added a waveclear/splitpushing/even faster roaming playstyle to Taliyah's kit, making her, again, more generalist. This made it ridiculously easy to just Q through an entire wave and permapush to turret and then roam bot for ganks and kills.
Coincidentally, with all these new unnecessary additions, she did well in LCS. Hence, her entire kit being nerfed into utter trash as a result of Riot's own stupidity.