I only ask because whenever I use him I seem to have strong games with him. His burst is nice and movement speed is good and hes lethal if you save your R for the right moments.
Hes not weak hes just weak compared to the 3 crit items rush adcs
Because he doesnt use ardent well which everyone wants to build. So he goes and becomes top tier in a different role like the other caster adcs 
hehe
He's a niche adc and 100 timers better at it than Lucian or ezreal.
Ever since the Triforce changes, his core item no longer has crit. That along with crit items becoming cheaper pushed him out of the bot lane. He relies too heavily on burst/spells which makes him a better mid. He basically became off meta in the marksman update then pushed out as the meta shifted towards hypercarries and late game crit carries in general. Corki joins Lucian and Ezreal in having a really nice early/mid game power spike but falls off later, making him better mid than ADC, especially with his package roaming potential.
He's very level dependent so he's better in mid where he's solo.
