Why is AD Kennen so useless

Hello, back in season 7 I was really successful with Kennen ADC pick. And in preseason he just got buffed only his healths. I just want to know why Riot Games don't want AD Kennen or even more ADC Kennen. I remember when they nerfed AD Kennen, that Riot want to keep Kennen on ADC. Quite interesting. It's really funny to watch at least for me.
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