The New York Times will later note, “There appears to have been no prohibition on the exclusive use of a private server; it does not appear to be an option anyone had thought about.” (The New York Times, 8/8/2015) But the State Department requires that computers be officially certified as secure, and no evidence has emerged that Clinton’s server was given such a certification.
Additionally, the department’s Foreign Affairs Manual (FAM) states, “Only department-issued or approved systems are authorized to connect to department enterprise networks.” (US Department of State)
One reason Clinton might want to use a private server is that the State Department computer systems at the time are widely considered inadequate and frustrating. One result of using a private server is that only a small fraction of emails used on the department’s systems will be permanently archived. (The New York Times, 8/8/2015)