As a factual matter, the Privacy Act generally prohibits the disclosure of personal information of whom the Government has collected information.
Further, the Freedom of Information Act contains exceptions, including:
On first blush, I would see that an investigation into a person would generally involve looking into personal matters, and the law generally seeks to not throw that out there without that person’s consent.
I think the fact that it is not being made public applies to all the background checks the FBI does, not just this one - at least as far as SC nominees goes. None of Kavanaugh’s previous background check are available for public viewing. (To be clear, I’m not talking about this report being in a secure room, that’s probably so there’s a lesser chance of it leaking). The basis appears to be that’s just the way it’s done.
And there is very good, practical reason for the state to keep these confidential-- to ensure people are willing to testify. Remember, no one is required to answer questions in an FBI background check. Promising confidentiality ensures that some folks who would otherwise not agree to testify, or who might alter their testimony if they knew it was made public, will cooperate with the FBI. In this particular case, the only alleged eye witness to the Ford allegation, Mark Judge, agreed to testify on the condition of confidentiality. Would we prefer a report without his testimony?
The exemption to the FOIA that Ravenman linked to makes complete sense.