this is not an easy question to answer. i don't even think there is just one answer. some people may have had a bad experience with adoption in the past. some may have misconceptions about what adoption is all about. some people are well aware of what adoption entails, and still feel it is not right for them.
everyone just has to do what they feel is best for them, and as with many other things in life, not everyone will understand their rationale.
p.s. congratulations on your son and good luck with your future adoption!