The Vilna Gaon says that Mordechai knew that Esther, being a righteous woman, never voluntarily submitted to Achashverosh carnally. In addition to the fact that he was a gentile, we learned earlier that Esther was married to Mordechai before she was forcibly removed from his home. Accordingly, Esther would need the force of a command to submit to Achashverosh voluntarily. There is a story told of a community rav who was in a situation in which circumstances were such that he had to build a synagogue where a mikvah once stood. Otherwise, his congregation would have no home. Knowing that Halacha (see Igros Moshe, Choshen Mishpat I, Siman 40) does not allow for a shul to be built in such a place, he asked the Chazon Ish for advice. The Chazon Ish reportedly told the rav that he was right, and that building the shul in such a location would earn him punishment in this world and the next. Nevertheless, he still had to do it. His congregation needed a home, and, as a leader, he had the responsibility to accept punishment for their benefit. Here, too, Esther was required to perform this sin for the benefit of the entire nation. Esther would not have gone to Achashverosh voluntarily.
R’ Moshe Dovid Valle quotes the verse in Koheles (3:7) that there is “a time to be silent, and a time to speak.” In other words, Mordechai was telling Esther that there was a time when he commanded her to remain silent regarding her ancestry (see 2:10); now, he was rescinding that command and telling her to speak.
R’ Dovid Feinstein gives another reason why Esther needed to be commanded. Quoting a Rashi in Vayikra 6:2 (in Parshas Tzav), he writes that the word, “tzav” (“command”) is only used when the person performing the action is reluctant to do it because there is something they stand to lose. Here, Mordechai has to command Esther because he realizes her self-sacrifice. Recalling that Mordechai is speaking to Hasach (Daniel) that he has to command Esther because he is a greater Torah scholar. As such, Esther would be more likely to listen to the command.
According to Targum Sheini, Mordechai was concerned that the king would become upset with Esther at some point, and take his anger out on her people. This is similar to what actually happens when Haman becomes angry with one Jew, Mordechai, and decides to exterminate his entire people as a consequence (see below 3:6).
The Ibn Ezra quotes commentators critical of Mordechai for this order, saying his selfish intent was for Esther to not be chosen by Achashverosh, so that she could return to being Mordechai’s wife. He rejects these opinions, and comments that Esther’s keeping this secret allowed her to stay Jewish because the king would have used violence in an attempt to force Esther to convert had he known her background.
Rabbi Eliezer of Worms writes that Mordechai knew that Esther’s very presence in the harem of the king meant that she was placed there for a reason. After all, wherever we are in life, H-Shem wants us there for a reason, though we often do not readily know what it is.
Rav Moshe Meir Weiss adds that even if Esther did not understand the reason for her secrecy, or even if she disagreed, this verse is a praise to her for listening to the words of Mordechai, the Gadol HaDor (greatest rabbi of her generation). When we trust the Sages, things go well for us.