Because it would be breaking privacy law to do so. It would also be breaking privacy law to comment on the content.
The only one that can resolve this issue, absent a court order in the public interest, is 0bama himself, who can release the document to the public.
Except that some Hawaii state officials have been, at least allegedly, quoted as saying all is right with Obama’s long form.
This has added to the general perception that there is no “there” there.