Due to Kyurem-B's limited Physical Movepool, he was put down to OU. He commonly has to rely on being a Mixed attacker, which is a let down, since he is putting the godly 170 Base attack stat to waste. He usually runs scarf, so when locked into a Physical move, Ferrothorn can Easily come in and sponge an attack.
Kyurem-W however, has exellent coverage in Ubers. Fusion Flare + Draco meteor + Turboblaze gives it perfect coverage, has a higher Base Special attack stat than Reshiram, and there really aren't many things that can take one of its STAB moves. (except for the Blobs)
In conclusion, it's really only due to the fact that coverage is important in Ubers, the waste of a tremendous blessing since Kyurem-B doesn't have a real good physical movepool that made Kyurem-B fall to OU. Kyurem-B can be used in Ubers anyway, if that's what you feel like doing.