I had read elsewhere that Rice and Powell did similar things during their terms. If you prosecute one then you have to prosecute the others, otherwise it's purely for political reasons, though I don't know if the previous secretaries' malfeasance rose to the same level as Clinton's. Also, a conviction would be very difficult to get if Clinton could show that she and/or her staff were misinformed or uninformed about particular regulations and any changes to them (to accommodate new technology, more security, etc.).
Shorter Comey: Dagnabbit, we almost had her!