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.