The Halacha is that those who live in a city that was walled in the times of Yehoshuah reads on a different day (Rambam Hilchos Megillah 1:4-5).
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 Megillah on Yehoshuah the question again is why not just say from the begining of the capturing of Israel why mention Yehoshuah ?