Is the fact that she is a scaling AP champion in an early game role which provides less gold/exp than she requires.
You see this with numerous champs who are not viable in the jungle.
They lack the early game damage of champs such as Lee Sin and Graves who possess similar clear speed but greater damage resistance and greater early game damage.
It is why champions such as her will never see LCS play. The jungle, her only viable place, is a poor position for her.
It is also why champions such as Rengar and Kha'Zix can become so oppressive. They not only scale but possess large amounts of damage too.