User blog comment:Xilinoc/Ch. 665 - The Princess Dissection/@comment-24726378-20160310153033/@comment-2120712-20160310153508

The Hogyoku was an unknown quantity. Once Aizen merged with it and had it under his control, he was unkillable. My guess is that his restructuring is weak compared to the reality bending the Hogyoku does. In-fact the Hogyoku seems to take what Kisuke's bankai does and take it to cocaine fueled levels. Maybe his Hogyoku was designed off of his bankai?