Because when your consulate is used as cover for a CIA gun-smuggling scheme for Syrian rebels using Gaddafi's weapons, you don't use State Department email. Duh.
And if you get caught, you destory all evidence, keep your mouth shut, and hope it blows over.
In January 2014, the Senate Intelligence Committee released a report on the assault by a local militia in September 2012 on the American consulate and a nearby undercover CIA facility in Benghazi, which resulted in the death of the US ambassador, Christopher Stevens, and three others. The report’s criticism of the State Department for not providing adequate security at the consulate, and of the intelligence community for not alerting the US military to the presence of a CIA outpost in the area, received front-page coverage and revived animosities in Washington, with Republicans accusing Obama and Hillary Clinton of a cover-up. A highly classified annex to the report, not made public, described a secret agreement reached in early 2012 between the Obama and Erdoğan administrations. It pertained to the rat line. By the terms of the agreement, funding came from Turkey, as well as Saudi Arabia and Qatar; the CIA, with the support of MI6, was responsible for getting arms from Gaddafi’s arsenals into Syria. A number of front companies were set up in Libya, some under the cover of Australian entities. Retired American soldiers, who didn’t always know who was really employing them, were hired to manage procurement and shipping. The operation was run by David Petraeus, the CIA director who would soon resign when it became known he was having an affair with his biographer. (A spokesperson for Petraeus denied the operation ever took place.)
The operation had not been disclosed at the time it was set up to the congressional intelligence committees and the congressional leadership, as required by law since the 1970s. The involvement of MI6 enabled the CIA to evade the law by classifying the mission as a liaison operation. The former intelligence official explained that for years there has been a recognised exception in the law that permits the CIA not to report liaison activity to Congress, which would otherwise be owed a finding. (All proposed CIA covert operations must be described in a written document, known as a ‘finding’, submitted to the senior leadership of Congress for approval.) Distribution of the annex was limited to the staff aides who wrote the report and to the eight ranking members of Congress – the Democratic and Republican leaders of the House and Senate, and the Democratic and Republicans leaders on the House and Senate intelligence committees. This hardly constituted a genuine attempt at oversight: the eight leaders are not known to gather together to raise questions or discuss the secret information they receive.
The annex didn’t tell the whole story of what happened in Benghazi before the attack, nor did it explain why the American consulate was attacked. ‘The consulate’s only mission was to provide cover for the moving of arms,’ the former intelligence official, who has read the annex, said. ‘It had no real political role.’