Self-interest has much to do with decisions that should be made disinterestedly. The social-services people for Virginia reviewed the findings of Child Protective Services, on which the conviction was based, decided the children's testimony had been manipulated, and determined the charges to be unfounded. Now, you or I would think this reason enough to get un-manipulated testimony from the kids. Thirteen years is a lot of a guy's life to take when things aren't clear. The prosecutor wants to let the conviction stand.
Human nature is a real good guess.
Imprisoning the innocent is not part of the job description. Prosecutors are promoted for winning cases, not for embarrassing the state. Many have political ambitions, which is fair enough. But an opponent in an election would make much of highly publicized professional mistakes. It's easier to say, well, he's probably guilty, and hang tough.