Esther 2:10 (ESV)

10 Esther had not made known her people or kindred, for Mordecai had commanded her not to make it known.

Mordecai and Esther were not the strong heroes of faith that we know Daniel and his friends to have been. Daniel and his friends had refused to eat King Nebuchadnezzar’s food, as God’s law had forbidden it. Esther, on the other hand, could not refuse any food that was offered her, since Mordecai had told her to hide her Jewish nationality.

Mordecai probably had a foreboding of the trouble that was brewing for the Jews. Just like his father Darius, Ahasuerus also received written complaints about the Jews who lived in Jerusalem and Judah (Ezra 4:6; Ezra 5:6). The worst persecution was about to be initiated by Haman (Esther 3:1–15). Thus, from a human perspective it was probably safer for Esther to hide her identity. Mordecai may have also had in mind the possibility of Esther’s becoming queen and acting on behalf of her people.