User blog comment:Godisme/The Gotei 13 repeat and reboot wrap up/@comment-44061-20120202055956/@comment-1393830-20120202144737

If I had to take a guess, I would say he likely chose to not have it restored out of pride. To him its a battle scar. If a situation arises in which he needs to step into combat, he will likely have his arm restored but I am thinking that the arm is his battle scar.