Because of the inhabitants of the land - These were the Ammonites, Moabites, Idumeans, and the remains of the Midianites and Amorites. But could the women and children even keep the defenced cities, when placed in them? This certainly cannot be supposed possible. Many of the men of war must of course stay behind. In the last census, Numbers 26, the tribe of Reuben consisted of 43,730 men; the tribe of Gad, 40,500; the tribe of Manasseh, 52,700; the half of which is 26,350. Add this to the sum of the other two tribes, and the amount is 110,580. Now from Joshua 4:13 we learn that of the tribes of Reuben and Gad, and the half of the tribe of Manasseh, only 40,000 armed men passed over Jordan to assist their brethren in the reduction of the land: consequently the number of 70,580 men were left behind for the defense of the women, the children, and the flocks. This was more than sufficient to defend them against a people already panic struck by their late discomfitures and reverses.

Numbers 32:16,34 And they came near to him, and said, We will build sheepfolds here for our cattle, and cities for our little ones...