I’ve explained it several times.
It’s called law enforcement privilege.
www.justice.gov
It is the policy of the Executive Branch to decline to provide committees of Congress with access to or copies of law enforcement files, or materials in investigative files whose disclosure might adversely affect a pending enforcement action, overall enforcement policy, or the rights of individuals.
Congressional assurance of confidentiality cannot overcome concern over the integrity of law enforcement files, not only because of concern over potential public distribution of the documents by Congress, but because of the importance of preventing direct congressional influence on investigations in progress.
It is the constitutional responsibility of the Executive to determine whether and when materials in law enforcement files may be distributed publicly, and this responsibility cannot and will not be delegated to Congress.