In 2013, Sandra announced she would be leaving Grey’s Anatomy the following year. After working on the medical drama for nearly a decade, the actress felt it was time for her and Cristina to try something new.
“Creatively, I really feel like I gave it my all, and I feel ready to let her go,” Sandra told The Hollywood Reporter in 2013. “It’s such an interesting thing to play a character for so long and to actually get the sense that she wants to be let go as well. [Cristina] wants to be let go, and I am ready to let her go.”
Source: https://www.distractify.com/p/why-did-sandra-oh-leave-greys-anatomy