Is it explained anywhere how the Nine tails biju was kept in control by all its jinchuriki?
Hashirama's wife, Mito and Kushina both belonged to the Uzumaki clan. It's as Kabuto said, the Uzumaki had life forces “as tenacious as a cockroach's”, because they were able to survive situations which would otherwise prove fatal for other people: Having a large amount of chakra was never what the Uzumaki were known for. They were always revered for their vitality.
The Uzumaki were also extremely skilled in sealing jutsu. In fact it was Kushina who taught Minato about fuinjutsu(along with Jiraya). All this enabled the Uzumaki to behave as the perfect Jinchurikis.