I don't know why they would do that, maybe it never crossed their minds. But I do know how the girls are placed in each game. As we know, different idols have different strenghts and weeknesses. The Idols with higher vocal stats are in Missing moon, the visual style idols are in wandering star and the dance idols and/or well rounded idols ar in perfect sun. The rival idols act as foils for the playable idols. EX.
Hibiki is the best dancer in 961, she's in perfect sun.
Takane is the most mature vocal wise, but if shes the best is your opinion, in 961 and is in wandering star.
Miki is definatly the most visualy noticable, with her blonde hair and over all style and shes in missing moon.