The Maharsha says that, as we shall see in the next answer, the party was meant to endear Achashverosh to Esther specifically for her to divulge her lineage. It was therefore named after her. The Me’am Loez reminds us that “Esther” also means “hidden,” and this feast was thus meant with the express purpose of her revealing her secret identity.
Also, the above-mentioned concern for the poor also makes this uniquely “Esther’s” party.
The Ben Ish Chai adds that this party demonstrated to the people conclusively that the icons of Vashti were down, and the king’s heart belonged wholly to Esther.
Rav Chadidah says this party was made by Achashverosh for Esther, as opposed to Vashti’s party, that she made for herself (1:9). Rav Chadidah points out that this proves a great affection the king had for his new bride.