Go-Go Enslavement and Bathe Her and Bring Her to Me both are based on the idea that enslaving a beautiful woman is exciting. It's about dominance and power being combined with sex which apparently turns some people on. (However it's not accepted behavior- so it's usually the villain who engages in it). A girl who is born a slave and then grows up to become beautiful is instead about the contrast between her beauty and her circumstances. "A lily among the weeds." (Never mind the Unfortunate Implication that other slaves are all weeds, or that beauty is a measure of a girl's worth, and that it's okay to enslave ugly girls.") Because of this it's more likely to be the setup to a Rescue Romance. There is also more likely to be emphasis on the girl's purity as well. And the idea that goodness and beauty can be found anywhere, not just in prosperity. Examples:
Hello, Unknown Troper. You'll need to get known to lend a hand here.