I could see a general cultural unity coming about during the Cetagandan invasion; nothing like all-out war and us v. them for unification. If it happened that recently, that could also explain why the language divisions are still extant. It seems less likely that the cultural divisions that tend to follow language would have been that thoroughly erased, though.

We may just have to posit, in-universe, that none of our narrators has taken much notice of specific cultural practices, though that's really a bit of a stretch.

Out-universe, I think it was an oversight.