The key factors are:
- loss of federal monetary support if the foster child is adopted. (which makes a family unable to adopt a children unless they belong to upper economic bracket)
- private insurance companies are reluctant to cover preexisting conditions.
- loss of federal support for medical expenses if the foster child is adopted. (combined with the relucatncy of the insurance companies, this often makes a family feel that taking a children from the foster will carry much greater risk compared to the reward that they would obtain)