This has to do with the concept of the Big 3. For a little bit of background on what the concept of the big 3 is you can take a look at What phenomenon are at play such that the "Big Four" are limited to four and not a growing list?.
With the ending of Naruto in sight at the time, and the great reception of Boku no Hero Academia (BNHA) at the ...
The Byakugan actually originated from the Ōtsutsuki Clan. As revealed in Chapter 671 and as stated in the wiki, the Byakugan
originated from the Ōtsutsuki Clan and was later inherited by their distant descendants, the Hyūga Clan.
Outside the leaf village, I can only recall Ao as the known possessor of the Byakugan but it should be noted that this was ...
He has extracted the Bijuus with one to seven tails but the former Jinchuuriki of the Ichibi - Gaara - is still alive.
Another reason is that he is controlling them using the Six Paths of Pain
an Outer Path technique that allows a Rinnegan user to manipulate up to six bodies as though they are their own.