For 6 it's not really an oversight, I just can't think of any use case from enabling that expansion mid-game. Nomads and the Exodian Blade can't be enabled mid-game because they rely on mapgen logic. About the only thing it would do is make the AI's future unlocks be able to include the new bonus/guardian types. The ship types associated with ARS's are already set.
For Expansion 5 I don't remember if the omission was intentional, but the same question basically applies: what's the use case?