User blog comment:Godisme/Everything But The Rain/@comment-1747976-20130304102648/@comment-1489595-20130304135317

If you want to believe Aizen's inferences then it was because of his manipulations and the power of the Hogyoku that Ichigo maintained his Shinigami power long after it should of fled back to Rukia.

It does provide a plausible explanation (as implausible as Aizen is...lol).