Consider the financial situation of a household which adopts children - adoption is a very expensive and rigorous process, so adopted children of any nationality are going to be in a family which is financially secure and meets some kind of standard psychologically and legally. Not so for many Asian immigrants - I would imagine that on average, Asian immigrants aren't as well off as foster parents. Culturally, their environment will be quite different.
Cultural identities also come into play - adopted or not, Asian children will identify themselves as "Asian", just as Jewish children identify themselves as "Jewish", Russian children will identify themselves as "Russian", and African American children identify themselves as "Black".