User blog comment:Majestic Soaring Cloud/Injuries in Bleach/@comment-8295743-20130708045530

This isn't an answer that's manga-canon. It may or may not be anime-canon, the last filler arc isn't really referenced in the final season at all. I say it may be anime-canon, since the first filler season *was* mentioned a few times in the proper anime.

Anyway. During the Gotei 13 Invading Army arc, Inaba was fighting Yamamoto and talked about his missing arm, saying the Old Man wouldn't let his pride accept help from Orihime to allow his arm to be restored. So, it's possible Orihime was the one to restore everyone's lost limbs.

An answer more manga-canon: The sacrificial spell that caused him to lose his arm, Itto Kaso, may have prevented his arm returning as some weird-side effect. It's a Forbidden spell for a reason, maybe that's why.