The Alliance was there to take Lordearon. Arathi Highlands warfront took place with Sylvanas not even on the continent. Anduin and Genn may have wanted to bring Sylvanas in to answer for her “warcrimes” but there was always a chance that that would not happen. A huge assault on the Undercity was bound to end in Sylvanas’ death and massive loss of life on both sides. Genn knew as well as Sylvanas and Saurfang that the Undercity was lost the moment the flames hit Teldrassil. Human soldiers believed they were retaking their human birthright.