Due to the advantages that Kyurem-S had of having a better Special Movepool, it complimented White Kyurem and its montrous SAtk and Ability. Black Kyurem received the short end of the stick Movepool wise, which is a reason of which it dropped. Kyurem-B also doesn't have the Speed to fill as a weather counter or Moves to be an abuser of it(White Kyurem does abuse weather with Fusion Flare).
