Toneri needed a pair of pure Byakugan to awaken the Tenseigan.
Within the Hyuga Clan, only Neji and Hanabi had Byakugan pure enough to meet that requirement.
But Neji bore the branch family's Caged Bird Seal, leaving Hanabi Hyuga as the only choice.
[For Toneri's origins, see the tenth Naruto movie, The Last. Also, regarding the earlier mention of "Yomotsu Hirasaka," it requires "both" the "Byakugan" and either "Six-Tomoe Rinnegan in both eyes" or "the ocular power of the Rinne Sharingan." Refer to "Kaguya Otsutsuki" and "Urashiki Otsutsuki." Is that sentence so hard to understand? Why do some people keep missing the point, forcing me to go over the wording again and again?]
In the shrine on the moon, Toneri and Black Zetsu spoke face-to-face.
Toneri said, "Hinata, my future wife, already possesses the ocular power of the Tenseigan. She's extraordinarily powerful. How am I supposed to go after her younger sister, Hanabi?"
Black Zetsu said, "You needn't worry about that. I have a hidden trump card in Konoha. He'll create chaos there, and things will escalate until Hinata has to intervene personally. You can make your entrance amid the chaos, take Hanabi Hyuga away, transplant her Byakugan, and awaken the Tenseigan you've always dreamed of!"
In the dim candlelight, Toneri turned his empty eye sockets toward Black Zetsu and asked, "Why are you helping me? What do you want from me?"
Black Zetsu knew how to tailor his approach to his audience, and he knew exactly what to say to earn Toneri's trust.
Having survived his clan's internal strife as a child, Toneri would never trust help offered for no reason.
Only negotiations that laid out the intentions directly, then explained the benefits and drawbacks, would put Toneri at ease!
So Black Zetsu spoke frankly. "Heh, heh. Then I