Why do people not play quin

Because we have souls

Because you can pick someone actually useful like a bruiser or tank.

Cuz she's only picked by "mains" or people that wana fuck over sm1s day. There are more fun and stronger picks... Plus its quinn.. Fuck quinn..

She actually does fall off late game when everyone has armor and her vault doesn't have the same power it did at the beginning.

She is a slow champion on foot and if the game lasts too long your ult is kind of worthless except if you want to try to rush a backdoor. It takes too long to channel to use as an escape. Eventually you end up just being a cleanup champ and are pretty useless in teamfights.

The main reason why, is because unless fed, shes not all that great. Mid game she is her spotlight, where she ganks nonstop winning lanes. But most games in S6 end in Late Game, where she is pretty much useless. The only thing she can really do late game is blind ADC in team battles, and the jungle warding quickly. But why have her do it when you can get teemo to do it? Teemo's blind is not a skill shot either, so it will hit every time.

Shes not very attractive

Try playing her on Doom Bots right now. Her late game weaknesses will become apparent to you.

She's designed to have a strong early mid game, destroy her laner and split to victory. She doesn't fall off in damage, however she does fall off in terms of out putting her damage because of her low range, which is a common misconception. Hence why she doesn't team fight, you want to assassinate carries who are out of position, she is really good at punishing them.