Because the Hyugas are the Oldest clan Kahona has to offer, a character already mentioned it, i forgot who though and on what Manga chapter, and there would be no way the Hyugas are decendants of the elder brother, because the Uchiha's eyes specializes in Ninjustu and Genjustu along with copying the Movements of the opposite user, and recording the Nin and Genjustu the rival uses, and the Mangekyou, and the Hyuga's specializes in Taijustu, sensory and defensives against other users, along with th Fact that the Byakugan has no set backs eccept the blind spot that can be fixed, the Sharingan's nature and Byakugan are too different to be related, too many prices must be paid for the Sharingan to reach full power, prices that makes you turn on a loved one or worse, thats why the hyuga's CANNOT be decendants of the Uchiha.