User blog comment:RoyalGuard-Elite/Kenpachi Shikai/@comment-9365367-20130914170247/@comment-5398287-20130914214442

It was always in Shikai because of Kenpachi's raw power. Ichigo is a special case because he never had a real Zanpakuto to begin with - the "Shikai" he gained during his training with Urahara wasn't a real Zanpakuto.