Science fiction, as a genre is fundamentally about ideas. It's about asking an impossible question, "What if...?" and building a story out of the answer. Romance on the other hand, is fundamentally about relationships. The hypothetical romance transposed to the past could be rewritten without the futuristic elements and still work as a story, which is something that can't happen with SF. It works in romance, because the story is the relationship and that depends on character, not setting. Lots of books take elements from multiple genres, and there are elements that put them into one genre or another, but setting isn't a key determinant.