Reshiram/Zekrom represent creation, a future to fight for, Kyurem represents coldness of absence, a regression towards the past with it being the supposed husk of a bygone dragon
Does the void care what differences there are in the abstraction of truths and ideals, futurism and tradition? They don't actually clash, they can coexist and be interchangeable with each other
Kyurem isn't absence or wuji itself since it is an actual, usable pokemon. The pokemon that literally does not exist beyond passing references for over a decade is the original dragon. Kyurem is yin (cold, darkness) to Reshiram/Zekrom's yang (creation, light, heat), further exemplified by Kyurem able to absorb the other dragons, but only one at a time (intentional because interchangeable). Kyurem's forms are yin-yang realized, contradictory forces of the past-future, death-life, literally frozen over fire/electricity as one entitiy.
Kyurem underwent a new transformation rather than regressing to the original dragon is summarized by N's plot of fighting for his ideals/uncovering the truth with one of his final words in BW2
>I learned something important. To make the world better, you must accept different ideas.
I learned that this is the formula for changing the world. Accepting different ideas… I want to see if you a Trainer whose heart is strong enough to do that.
The world is not static and not black and white. N and Kyurem started and ended in a different places from where they started, and is a part of what Unova (literally based on the US and specifically NYC) was going for - leaving the old world behind, start on a new beginning of the franchise, taking bits and pieces of the old world and making something new of them
