Oznayim la'Torah #1: Because, as opposed to the B'nei Reuven and Gad, who all came together to inherit, even the two families of Menasheh - that of Machir and of Ya'ir - who came to inherit in Eiver ha'Yarden initialy came indipendently, ¹ and others joined only later.

Oznayim la'Torah #2: According to the Ramban at the end of Matos, who proves that Ya'ir was actually from the tribe of Yehudah, and it was his mother who was from the tribe of Menasheh, it is obvious as to why the Torah omits "le'Veis Avosam" by the tribe of Menasheh.