The most likely reason is that the the emails contained discussions--even negotiations--about her sale of State Department favors for contributions to the Clinton Foundation. This HAD to be done on private servers so they could be destroyed if sought by Congress or the DOJ.
I have NO doubt that she 100% intended to squeeze every red cent out of her job as Secretary of State that she could manage. It was pay to play .... it had to be kept private. Catherine Herridge, one of the few true reporters out there, has stated that the FBI has recovered the “wiped” emails - I am sure the FBI has plenty to work with regarding “corruption” charges. In fact, I’m sort of wondering if the McAuliffe investigation didn’t either come about or get legs due to emails Hilly had tried to delete ... just a thought.