Why were two episodes from Season 10 (2005) left unaired?
Season 10 by the way, was the last season of the Lisa Foiles/Jack DeSena/Chelsea Brummet/Kyle Sullivan era (the first relaunch cast) and the last season of All That until the 2019 reboot.
https://lostmediaarchive.fandom.com/wiki/All_That_-_Lost/Partially_Lost_Episodes_(1994-2005_Nickelodeon_Series)
The two unaired (at least in the United States; they aired in Australia) episodes had Bow Wow and Brie Larson (yes, Captain Marvel herself) as musical guests.
https://nickelodeon.fandom.com/wiki/All_That_(season_ten)
https://en.wikipedia.org/wiki/All_That_(season_10)#Unaired_episodes
https://animesuperhero.com/forums/threads/nick-has-lots-of-unaired-episodes.5423601/page-3