And if you look back for search results on Sasha Walpole before the news broke there's hardly anything. Why would she wait 20+ years to sell her story, why not sell it when she was younger, or when the book was released?
She seems to be a a woman who has lived a pretty regular, and quiet life out of the limelight. There's no reason for her to come forward at this point, unless like others have said the story was going to leak. At least this way she has some control over what is being said.