I believe Momo was prohibited from attending the festivities on the station.
This may beg a question: why?
...That's not how you use 'to beg a question'.
I asked why Momo wasn't there, and jwhouk posted an explanation which left open the question of "why". I believe that is a correct use of "to beg a question", and not the popular misuse where it's used as a way of saying "to invite a question".
That also leaves the question of why Marigirl hasn't taken Momo to planetside social events like the Horrible Revelation excursion.
Perhaps consumer-grade AIs are too flaky for a controlled environment. Imagine Momo having a tantrum like the one that blew up Marigirl's gaming rig, but near an important piece of equipment. One would hope that all the _really_ important machines are behind locked doors, but we can be confident that nothing there is cheap.