Board Thread:Theories and Speculation/@comment-24824859-20141204001040/@comment-945885-20141208184418

According to the databook, the Byakugan was nearly "bred out" of existence after many generations of Hamura's bloodline being diffused. The Hyuga Clan recognized that and dedicated themselves to preserving it, which is why they still have it.

Toneri was blind before he took Hanabi's eyes. The Otsutsuki Clan has been sacrificing and sealing their eyes into an altar in order to accumulate power for a thousand years. That power is called the Tenseigan (Reincarnation Eye) and represents the stored up ocular power of some 40 generations of Otsutsuki Clan members.