Because playing her is very... special.
She doesnt have the best laning phase... but she's one of the best minion/wrait farmers out there.
She has a snare and a slow... but she's extremly easy to gank and kill.
She has a high damage potential... but may die instantly if you pick the wrong momment/spot to unleash it.
Her snare is AoE and deals high damage... but is one the trickiest skillshots to land in the game.
She fully relies on fast/good positioning and medium range skillshots to play... But she has one of the lowest base Hp in the game (355+74 per level... only higher than Sona's) and 300 movespeed.
She's a sustained damage champion with peaks of burst that needs loads of mana, AP and CDR, so there isnt much room for survavility.
Get it? As Zyra, you pay very hard for mistakes but your gameplay revolves around doing the things that are the hardest for you. Every nerf was made to counter her actual gameplay and made her much harder to play as.That's why she's so underplayed yet so powerful. If you decided to embrace the nerfs and play her as hard as she is you will end up having sucess, but her unique and rare play becomes annoying for people just "trying her"