Responses

Because she's the only reincarnation? The others aren't reincarnations of the previous dragons or anything. They're dragon warriors, who just always happen to be born in the same tribes. But there's no soul being repeated, those are fully separate people.
In this case, she is a reincarnation of the first king, who was the crimson dragon encarnate. Souls don't choose the bodies they land in, they just are.
was it ever explained why the red dragon was the only one who reincarnated as a woman?