User blog comment:Godisme/The Gotei 13 repeat and reboot wrap up/@comment-1933234-20121023032042/@comment-1393830-20121023032807

The reason Kenpachi is a captain was explained a long time ago. He killed the Kenpachi before him in a fight in front of 200 witnesses, thus meeting the criteria of one of the 3 methods of becoming a captain.