It took me a few chapters past the point where the characters started losing me to figure out why. I read scenes and knew I ought to feel much for the hero, but the distance between reader and character was too much to ignore. At last, I figured out why.
Even as much as the plot’s setup is interesting, the main characters lack troubled pasts. Certainly, they went through the normal parts of life, but I lost sympathy for them. Like Mary Sues, their lives were too perfect. Yes, they had experienced some normal sorrows, but no more than that before the plot began. I suspect, just as readers tend to dislike Mary Sues, readers also have difficulty relating to and enjoying a Mary Sue past.