Oznayim laa'Torah: Because now that many of the men over the age of twenty remained in Eiver ha'Yarden, ¹ Moshe advised them to reside in the towns that he had given them, which the men would be able to guard, rather than build fortified town at the edge of the Yarden to pretect their wives and children - as they had originally planned ² - which would have leaving the animals on their own for foirteen until the men returned after conquering the land and distributing it.

Ha'amek Davar: Because if not for the large amount of animals, which could not be left on their own, Moshe would never have allowed the men to leave their wives for such an extended period of time - he would have allowed the women and children to cross the Yarden into Eretz Cana'an together with the men ³ .