We have a Warrior of Light who is capable of going toe to toe with god-class entities that was super charged by the eye of one of midgarsormr's children. Not to mention hordes of people who rely on the Warrior of Light and place all of their hopes and dreams on the WoL's shoulders. We also not only managed to get the Blessing of Light back, but got back a supercharged version of the blessing.

Which by the way the eye has sufficient power to 1 shot ascians and power Primals.

It is more of a question why we did not turn into a primal as we beat the shit out of nidhogg, and less of why nidhogg lost.