Laws are for the little people, not them.
The Presidential and Federal Records Act Amendments of 2014 became law on November 26, 2014. Clinton's final day as secretary was February 1, 2013.
The "Law" that everyone keeps claiming that she broke wasn't effective until a year and a half after she left office.
There was absolutely no legal requirement at the time of her tenure to use a government e-mail. Furthermore, she retroactively complied with the records portion of the law by turning over any business related e-mails she had on her home server archive.
Also, previous Secretaries of State, like Colin Powell, used personal email as well. In his case, they didn't even archive it so many of the emails are lost. We'll never have access to his electronic discusssions about, say, the decisions leading for him to give a speech at the United Nations calling for the Invasion of Iraq.