"I do think social conditioning is stronger than we give it credit for. They had their own small rebellions, but none of them committing the ultimate taboo of refusing or just walking away. They were raised to accept their fate."
They were told they would be butchered for the benefit of others. No actual conditioning is described in the book. There is in particular no social conditioning, since they never lived in society but were always segregated from it. Nor is there any sort of attempt at an explanation of why on Earth they don't try to run away, or at least render themselves useless for example by doing drugs or getting infected with HIV, Hep B, etc.
This was one of my major problems with Never Let Me Go - clones' docility in willingly going to certain death made no sense. Even sheep would have better instincts of self-preservation and would make an effort to save themselves. In fact, I have seen sheep make more of an effort to escape slaughter once they start smelling blood. It looked like the author didn't know how to construct a a credible, internally consistent world and so just left it 
The "science" made no sense, either. Clones' organs are harvested randomly, it seems, and given to whomever. Which totally runs against all logic and frankly the only reason why anyone would go through the trouble & expense of cloning in the first place: To have tissue identical to your own, so that there will be no tissue rejection.
Sci-fi/speculative fiction gets looked down on a lot, but it is not easy to write a good book in this genre. The author must imagine a background significantly different from our own, with myriad little details that make up a consistent and credible reality for the story to emerge from. Ishiguro fails on all counts in this book and seems to know it, too, because he satisfies himself with tugging on heartstrings, which frankly makes really pathetic speculative fiction.