Because he did not apply.
Rabbi Korobkin was the runner-up to Steven Weil in the 2000 search for a successor to Abner Weiss. He did not throw his hat in the ring this go round.
By temperament, Rabbi Korobkin is more of a scholar than a pulpit rabbi. I believe he’s pursuing a PhD in Islam at UCLA.
Los Angeles is blessed with many great Modern Orthodox rabbis such as Korobkin (Elazar Muskin, Yosef Kanefsky — two pulpit stars of Pico-Robertson).