Because he only has like 400 range so it's hard to position correctly behind your backline and still do damage in teamfights with autoattacks.
IMO, his autos are weak compared to his Q + E combo. That's really all you need to be doing, especially in teamfights. They burn (E) and you can lock down ADC's with the lock-on AND slow them by about ~40% (W) [Which absolutely CRIPPLES high priority targets like ADC's like Jinx or Vayne], and they hit like a train, even more so with the Muramana (His ONE item he MUST build) active on. And that brings me to my next point,
Adding onto that, he's a mana hungry beast compared to most adcs that don't use much mana at all.
He DOES have early game mana problems, but once you have Muramana on him, he's good. (Assuming you don't keep the active on.) And I liked to build a Frozen Heart on him for the CD, mana, armour, and AA slow. An ADC's worst nightmare.
I just love this champ so much. Sure, he's less strong than other ADC's, but he's actually really fun.
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