Because your essentially transferring two Pokémon for one as, White Kyurem is a combination of Kyurem and Reshiram, and Black Kyurem is a combination of Kyurem and Zekrom.
So you must transfer them individually to Pokemon X and Y, then you can fuse them together again using the DNA Splicers, which you receive from an girl in Kiloude City when you talk to her with Kyurem in your party.
Hope I helped :)