I love books that cross the category boundaries. Unfortunately some people do not, and adding a bit too much romance to your sci-fi novel, or too-much sci-fi to your romance novel will likely piss off some readers of the genre and kill the rating, which career writers are forced to maximize for sales. The genres are forced on the book at the publishing stage and, unless you are an established author, picking the wrong one can be a disaster. (Been there, done that!)

I haven't read the novel, but going by the Wikipedia article you could argue that just because it features astronauts on the ISS doesn't yet make it science fiction - as long as nothing unusual happens to the astronauts, what goes on on the ISS is no more science fiction than what an airline pilot does in their day-to-day job.

Even the author seems to agree (also from Wikipedia): "In an interview, Harvey stated that her goal was to write the novel as a "space pastoral", focusing on realism and the beauty of space rather than the speculative nature of science fiction."

When you read Orbital, you very much get the feeling that it is squarely planting itself in the territory of "exploring right now in a literary way" rather than being sci-fi. I think it has a strong vibe of really not wanting to be considered sci-fi. That what makes it an interesting book

The distinction is that Harrison has spent his entire fairly lengthy career being published as a science fiction author, and the others did not. They were all firmly in the mainstream literary camp, even if their books occasionally strayed over.