While not trying to derail the topic of this thread, I understand your confusion. A majority of the time I have seen dark-skinned girls, they are not often in lead character roles but instead are lesser supporting roles. And they often are given characteristics that I can imagine may not be appealing to Japanese audience. I've seen the angry tsundere, the slutty seductress, the lazy/airheaded gag character, or (more often the case) protagonist's villain that does many bad things in the plotline. I can't name many anime series or even games for that matter that have a dark-skinned girls as the lead female protagonist. If there are any, they are vastly outnumbered by their pale-skinned peers. Maybe it also has something to do with the negative perception related to gyarus (specifically ganguro) that Japanese society has of them as far as my knowledge knows. But who really knows right?