Quote Originally Posted by Rane View Post
As others have said, it seems that Bahamut was put in a "temporary prison" that was meant to hold him as the 12 were summoned. Before the summoning could complete, however, Bahamut broke out of that prison and scuttled the whole thing.

I don't think it's that the 12 weren't able to hold Bahamut, but rather that they never got the chance to do their thing.
Dalamud had been absorbing aether for a long while, even weakening some of the primals. my guess is bahamut was juicing up on aether and possessed van darnus to draw his prison closer to the planet.