Other officials used private emails but my understanding is that it was still on government servers, which would hopefully be more secure than the private server that Hildebeast was using that was socked away in the bathroom closet of a mom and pop computer company in a Denver suburb. The ONLY reason for her to have a private server was so that the rest of the government, Bomma included, didn't know what she was up to.
Put it this way, if your stuff gets hacked on the government's computer, you aren't liable. If your stuff gets hacked and it's on your own ****, then you ARE liable. So, why would Hildebeast take that chance? 1) She doesn't know that much about computers, 2) she REALLY didn't want Bomma and Valerie Jarrett to know about the influence she was selling, 3) she didn't want Bomma and Valerie Jarrett to know that Sid Blumenthal (who they hate) was really the the one running State Dept. policy, 4) Hildebeast thinks that rules and laws are for the little people, and 5) all of the above.