Why wouldn't Mordecai bow down to Haman?

The chapter doesn't say, and this study won't guess. It tells what happened: "All the king’s servants who were in the king’s gate bowed down and paid homage to Haman, for the king had so commanded concerning him. But Mordecai didn’t bow down or pay him homage" (3:2). The servants asked him, "Why do you disobey the king’s commandment?" (3:3), and no answer of his is given. The chapter says only that "they told Haman, to see whether Mordecai’s reason would stand; for he had told them that he was a Jew" (3:4). In Hebrew, the WEB's reason there is the word Brown-Driver-Briggs gives as "speech, word", here plural: Mordecai's words. So the chapter tells you what he had told them, that he was a Jew, and it doesn't say how that bore on his not bowing. The book doesn't say more later; when it comes back to it, it says only that Haman saw Mordecai in the king's gate, "that he didn’t stand up nor move for him" (5:9). The chapter passes no verdict on his refusal, and this study doesn't either.