Perhaps its because Government servers are strong hacker and espionage targets.
The New York Times reports that Hillary Clinton used only her personal email (hdr22@clintonemail.com) address while serving as Secretary of State. As others have reported, a hacker calling him or herself “Guccifer” claims to have compromised the email account of former Clinton aide, revealing memos that Blumenthal purportedly wrote to then-Secretary of State Hillary Clinton about Benghazi and other matters.
Hilary’s habit of conducting official business through her personal email address came to light during the Benghazi investigation. As part of an inquiry into whether the former First Lady violated federal requirements, Clinton’s advisers handed over some 55,000 of her personal emails to State Department officials recently, but these were picked by those advisers, so it’s unknown exactly how large her personal email archive could be. It’s also unknown whether or not she used encryption or any security measures to protect those highly sensitive communications.