As I understand it, the big majority in foster care or institutional care are children whose parents are living and whose parental rights have not been terminated. These children are not available for adoption.
For those who are available for adoption, most of them are in the hard-to-place category, That means that they may be
- Older than toddler;
- physically or mentally handicapped
- members of sibling groups who would have to be adopted as a group
- victims of abuse who have emotional or psychiatric issues, e.g. drug addicted.
Newborn infants of any race, color or nationality (and almost any condition) are very readily adoptable. It's much harder to find permanent adoptive homes for older kids with multiple issues, especially behavior/emotional problems or juvenile criminal records.