After the seventy years of the Babylonian exile come to an end with the Cyrus declaration, a small number of Jews decide to return to Eretz Yisrael while the majority of Jews choose to stay in Babylon and Persia. The story of the Megillat Esther in Shushan takes place after Cyrus's declaration with those Jews who did not return. 

Courtesy of Rabbi Menachem Leibtag www.tanach.org