>>33462584This was somewhat explained in Kyurem's profile on movie's site though
>Kyurem: The strongest Dragon-type Pokemon, its power is on par with Reshiram and Zekrom. Carrying the strongest heredity, it was born at the same time as Reshiram and Zekrom. It can change into Black and White Kyurem. Extreme cold radiates from its body.I always thought it was simply for the sake of convenience; being able to tap into Reshiram and Zekrom's powers because it was born at the same time is a lot less convoluted than having to fuse with them, and it makes it easier to show off the forms and make Kyurem live up to its reputation as the strongest Dragon on Earth.
