SPOILER!!!!!!!
why did KISHI do it? i dunno lol. why I think they didn't have one (if you think about it from the story's perspective): team 7 got yamato and stayed team 7 because they were the only 2 left without a team, and they joined up with Kakashi, and Sai was added, but Kakashi had other stuff to do (i think), so Yamato came and filled in for him, as well as kept an eye on Sai and watch over Naruto.
As for team 8, they kind of lost their sensei in the middle of a crisis (ie right before Pein attacked), and right after, Tsunade went into a coma. I doubt Danzo would bother with that with all the Kage and other stuff going on, rebuilding the village, and the like. And when Tsunade finally woke up, it was all about confining Naruto to the island, right? and then there's the war, and here we are in the present. That's my take on it.
Oh, and I just thought of a reason for why Kishimoto might not write/draw in a new sensei. It would mean a lot of extra intros for no real reason. yeah, that's all I've got lol