Board Thread:Naruto Discussions/@comment-28581957-20170402195036/@comment-25319479-20170619155805

They had somewhat of an order, as the Nine-Tails was decided to be sealed last, otherwise the balance would be disrupting and damage the Demonic Statue.

Furthermore, White Zetsu and Madara acknowledged that there is indeed an order, as they both knew to seal the Tailed beasts starting from the 1-tail and ending with the 8/9-tails.

This order might explain why Obito despite having the 3-tails in his grasp didn't capture the beast, as the 1 and 2 tails weren't sealed yet.