Two reasons:
1.) It'll be easier to sell an all-boys IM@S game if one of the characters was established beforehand; i.e. a "Boy's Side" IM@S game would probably involve Ryo post-girl Idol trying to become more of a male idol (Advice, Ryo: apply for a role in a Kamen Rider or Power Rangers-type show. A lot of young male idols get discovered that way)
2.) Futanari have fans too. :p