Because then you wouldn't have the whole "outer appearance, inner hobby" conflict Kirino provides. A lot of people would be shocked to find out about Kirino, while most people wouldn't care about Manami, cute as she is.
I adore her. She's just so... Perfect. The manga version of her is a bit more amorous too. I wish there were some images of her with the dog ears and wagging tail she often has in the manga.