why was koro apirana unhappy that his eldest grandson and his wife had given birth to a girl?

By Maori custom, leadership was hereditary. It passed from the eldest SON to the eldest SON. Porourangi is the eldest grandson. Koro is desperate for Porourangi to have a son so that he, Koro Apirana, can groom the boy to take the mantle of leadership as tradition demands. Kahu is a girl, her birth dashes these hopes.