When Mordechai said, "Who knows?" he meant that perhaps the only reason Esther became Queen was for this very moment -- to save the Jewish people.
אמר ליה רב יוסף לרב יוסף בריה דרבה אבוך במאי זהיר טפי אמר ליה בציצית יומא חד הוה קא סליק בדרגא איפסיק ליה חוטא ולא נחית ואתא כמה דלא רמיה
Rav Yosef said to Rav Yosef, son of Rabba: In what area was your father, Rabba, especially vigilant? He said to him: It was in the mitzva of ritual fringes. The Gemara relates: One day he was climbing the stairs when a string from his ritual fringes was severed, and he would not descend until he placed a new string on the garment.
וזהו שאמר במאי הוי זהיר טפי טפי דייקא. והנה יתרון האור הזה הפרטי לנשמות פרטיות אינו בבחינת טעם ודעת מושג אלא למעלה מבחינת הדעת שכך עלה במחשבה לפניו יתברך ודוגמתו למטה הוא בחינת הגורל ממש:
Hence the word “especially” [used by the above-quoted individual] when he asked, “In what area was your father especially vigilant?” (For vigilance indeed applies to all mitzvot; the question was only, which command earned his keenest attention.)
Now, the superiority of this individual light [that is bestowed] upon individual souls through the performance of a particular commandment is not in the category of apprehensible reason,
but transcends it.
For thus it arose in G‑d’s thought, so to speak — [that certain souls be granted additional illumination through the performance of particular commandments].
And its model below is truly the concept of a “lot”.