The designer of each set makes a case for what figures they think belong in the set. They base it on what relevance that character has to the set, like you wouldn't put a gungan in a TIE Fighter set (sounds like some fun fan fiction, but for the sake of this discussion not so much). Its also a matter of how much value can they add to the set.
When they were originally designing the Mon Cal set, they didn't have budget for anything NEW other than the Ackbar fig, but the team was able to convince the people they were working with that these other characters were very important to the set in both value and role play.
the same thing goes for pretty much every set, and the designers are always trying to make their individual projects the best they can be.