The Halacha is that those who live in a city that was walled in the times of Yehoshuah reads on a different day. The question is why from Yehoshuah (Joshua) when the commentaries explain it they speak of the respect for Israel as the occasion occurred when the land of Israel lay in ruins hence we focus one aspect of the Meggilah on Yehoshuah the question again is why not just say from the begining of the capturing of Israel why mention Yehoshuah ?
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Bartenura (to Megillah 1:1) says that it is associated with Yehoshua because he was the first to wage war against Amalek. Indeed, G-d directs Moshe there to "write this as a memorial in the book and place it in the ears of Yehoshua" - the Gemara (Megillah 7a and 18a) explains that this phrase refers, among other things, to the Megillah.
This is derived on Megillah 2b via biblical exegesis. The Yerushalmi quoted by Rambam and others explains that this was to accord honor to Israel whose walled cities were destroyed in the time of the Megilla. By defining walled city status by the ealier periof of Yehshua, Israeli cities qualified. (The exact intent of the Yerushalmi is disputed by the commentators).