Powerful AIs are probably much more aware of their long-term goals and able to formalize them than a heterogenous civilization is. Deriving a comprehensive morality for post-humanity is really hard, and indeed CEV is designed to avoid the need of having humans do that. Doing it for an arbitrary alien civilization would likely not be any simpler.

Whereas with powerful AIs, you can just ask them which values they would like implemented and probably get a good answer, as proposed by Bostrom.