User blog comment:Godisme/Pure Speculation Thread/@comment-1853786-20100414125612

Question if everyone saw Aizen's Shikai at that one meeting or whatever so that they could be controlled I think maybe Kenpachi, Mayuri and possibly Byakuya haven't seen it. Think about it, it wasn't like they were required to see Aizen's Shikai by Yama or something and the only reason everyone else saw it is because Aizen was pretending to be friendly and stuff.

And I don't think Mayuri or Kenpachi would give a damn about some captains shikai and Byakuya may have been all like "Ima a noble this shikai business is beneath me".

That also may have been why Aizen made Rukia get executed on that day since Mayuri was regenerating from being turned into goop and the other two were tied up fighting people and stuff.

That may also be why Kenpachi, Mayuri and Byakuya are in Hueco Mundo.

I may be missing some critical fact in this speculation however.