Because Chyler and Shonda wanted to kill her. No, Lexie was going to help with the neuro of the twins they were separating. Derek had her in to assist another reason why he felt so guilt about her being there.
I think she was on the plane to help her sister Meredith out and also to learn and get lebih experience. It was sad when she died. I was REALLY upset when Mark died later on. He's my favorit character!