Ari Kahn: The Netziv suggested that the tribe of Menashe was not interested in staying outside Israel. Rather, Moshe asked them to do so because he knew that the other tribes would need Torah leadership. Menashe’s inheritance east of the Jordan was not allotted to them in response to their own request. Moshe initiated the move in an attempt to insure the integrity and continuity of the other two tribes.
Yair Barkai: Moses gave the half-tribe of Manasseh territory across the Jordan in order to be sure the Reubenites and Gadites would be prevented from breaking off from the rest of their brethren who inherited in the western part of the country; just as the goblet that Manasseh had placed in Benjamin’s sack, upon orders from his father Joseph, was intended to make sure the brothers were indeed completely united. In other words, the half-tribe of Manasseh served as the connecting link, joining the Israelite tribes on both sides of the Jordan.
