Not everyone could use the Rinnegan's techniques. They required a certain bloodline.
Nagato carried more than the blood of the Uzumaki Clan. He also had Hashirama Senju's blood in his veins and was suspected to be a descendant of Hashirama Senju and Mito Uzumaki.
The manga explicitly stated that Hashirama Senju's blood flowed through Nagato.
Madara Uchiha had chosen his Rinnegan vessel with great care.
Of course, Black Zetsu and the others were not trying to find someone who could wield the Rinnegan's full power. They only needed someone who could perform its Outer Path Secret Technique.
"Find me an Uchiha ninja!" Black Zetsu decided at once.
As Indra's descendants, the Uchiha Clan were the best candidates for performing an Outer Path technique.
Black Zetsu had waited a thousand years for a second pair of Rinnegan, for Madara Uchiha, and for a chance to break the seal of Planetary Devastation.
He refused to let that chance slip away and wait another thousand years.
Besides, Konoha's Ghost God had already entered the Six Paths Domain. His life had evolved, setting him on a path similar to the Ōtsutsuki Clan's, and no one knew how long he might live.
If he went a step further and achieved immortality, things would become troublesome.
At the Six Paths level, immortality was no longer a fantasy.
Worst of all, Kazan knew their plan. If Black Zetsu failed to seize this chance, even hiding might do him no good. The Ghost God would hunt him down.
They moved quickly and found an Uchiha ninja without a sound.
"Are we transplanting it just like that?"
"Yeah. We don't have time to wait for him to adjust, and we can't use Medical Ninjutsu."
Neither Black Zetsu nor White Zetsu was skilled in Medical Ninjutsu. They could not perform