Why is it Called Megillas Esther & not Mordechai?

From a talk by the Lubavitcher Rebbe at a Farbrengin on Purim 5733, March 18, 1973.

Mordechai and Esther are the heroes of Purim, yet the Megillah is named only after Esther. And the initiative that the Megillah be included among Torah Shebiksav came specifically from Esther. Why is Esther given the spotlight, when her success depended on Mordechai's plan?

The answer: Mordechai had the idea, but Esther took the action—and action is the main thing. It was Esther who actually risked her life to see the plan through. Esther wanted every Jewish woman in every generation to know their vital role. Their husband may be a great rabbi, a great leader—a veritable "Mordechai," but it is the Jewish woman who is the actual pillar of the Jewish People. It is she who establishes "family by family," "city by city," and "country by country," ensuring that even when the Jewish People are "scattered and dispersed among other nations," they remain "one unique Jewish People." 

Excerpt from Living Torah Volume 33, Episode 130.