We do know why it's sealed. Well, not specifically. But the judge said it could interfere with the investigation.
http://www.websleuths.com/forums/showpost.php?p=5362380&postcount=889
"After a hearing Friday, Kaine Horman's attorney, Laura Racknar, said her client needed time to review the request and file a response to the petition. Racknar presented an affidavit from the district attorney's office which stated that unsealing the order could have a damaging effect on the criminal investigation.
Judge Keith Meisenheimer said he's confident the public doesn't want to engage in the release of information that could compromise finding Kyron Horman."
Actually, that was the reason given for not unsealing per the media requests. Interesting that the DA did not appear. Kaine's attorney asked the DA to support Kaine in keeping it sealed and out of the public eye. The Judge's statement is based on the DA's affidavit. Even then the DA said it "could compromise."
IMO, if it really was sealed because it could compromise the investigation, the DA would be the one fighting to keep it sealed, not Kaine's attorney. IMO, that Kaine's attorney is fighting to keep it sealed supports that it was sealed to protect the minor child and not the investigation.