User blog comment:Xilinoc/Ch. 571 (Yachiru's gonna wreck it)/@comment-11446176-20140305130913/@comment-8669536-20140305181214

One can speculate all one wants, and the speculation may even be the likeliest option, but it is still that which has been stated by Kubo in the manga which is the law of Bleach.

Chapter 109, pages 16-17: Kenpachi says his Zanpakutou can't be sealed because of his overwhelming Reiatsu. It can't be sealed, thus it is in Shikai. It is logical.