User blog comment:Xilinoc/Ch. 580 - THE LIGHT/@comment-24284608-20140517134349/@comment-1489595-20140517182559

Its wishful thinking to assume that Kenpachi or Yachiru acquired bankai. Kenpachi was struggling just to learn his sword's name and get shikai. Yachiru...yeah...not gonna happen.

Besides the main appeal of Kenpachi isn't bankai. Everybody who is anybody in Bleach has Bankai. The appeal is that Kenpachi, with his wild, improbably powerful spirit power, can match and even exceed his peers WITHOUT Bankai. Giving Kenpachi bankai would destroy that uniqueness of character.