There is a reason why Juubito, after breaking the barrier, immediately went to Gamakichi's location:
And there is also a reason why Hiruzen wasn't targeted by Juubito:
It is because Hashirama and Tobirama weren't the true targets of Juubito, they were only getting in the way:
The true target is Naruto.
But why is that? why Naruto alone?
It is because of this:
The Juubi, controlling Obito, is after its original target: Naruto
The target truly was Naruto:
After the hindrances (Hokages) were taken care of:
Target locked on:
Sasuke intervenes, protects target (Naruto):