Maybe something happened to her parent(s)? Someone (in comments section) brought up the possibility that maybe her mother was abducted? Even so, you would think that someone else who knows the little girl/her family would come forward. But it is the 4th of July...how many people are watching the news? Maybe her single mother and her are driving through the country on their way to somewhere else, something happens to the mother, and no one in the area knows them?
It's possible she was abandoned, but that doesn't seem like a very well-thought-out plan if the family, especially if the family lives in the area. They have neighbors, family members, friends, classmates, etc who would recognize their daughter.