Here's why:
With the recent release of Dark Star Orianna, many avid Orianna players (myself included) emerged and began reminding everyone of how powerful Orianna can be. With Ahri getting nerfed and LB having been in the ground for a few months, people have gravitated towards a new mid-lane mage to complain about.
But here's the thing:
Orianna is not OP, and provably so. Her winrate is not outrageous, neither is her damage. Her CC relies on a powerful engage on her team (like Malphite or Vi), and against even a semi-mobile target her ball's damage output is neutered. She is more of a mage/support mix. She starts out as a decent lane bully but as the game progresses she transitions to a more CC-oriented, utility mage.
Admittedly, she can be frustrating to play against because it's easy to forget where her ball is and take easy damage. But that is inherent in her kit, not a flaw in design.