maybe they thot that it was the safest place for it...since rukia was like protected by byakuya so heavily.
i bet that from this time to the time of the hyoukyou(i cant spell it either) being made from where we r now, there's a big time since mayuri is still not helping out and it prolly takes them a long time to make it...
so by that time byakuya is prolly grown up and he met rukia's sister, then she died and he went and found rukia and yourichi decided to hide it in rukia(since she knows about it too prob.) b/c its the safest place
It cannot take too long, we know from the fight between Cat and Bee that Yourichi left the SS 100 years ago, and Yourichi left SS to help Uahara escape SS. and that this is 100 years in the past some big things are gonna go down really quick