Probably because his age did not allow him to realisticly portray his character anymore. Max Pirkis was around 15 while filming the first season, so with the help of changing clothes, he could play Octavian aging from about 11 to 18. However, during the filming of season 2, Pirkis had only aged about two years, while his character would age from 18 until around 32 years. So Octavian's absence from the episode These Being the Words of Marcus Tulius Cicero was used to introduce Woods (who was around 26 at the start) as 19-year old Octavian in the episode Testudo et Lupis.