Because there's no accusations, that I am aware of, of it being used to pass Classified information. This is the core issue with the Clinton servers, there IS evidence that it was used to pass Classified information AND in order to prevent proper investigation of how much, Clinton had the servers and more importantly the hard drives of them wiped and destroyed. Normally when people destroy evidence that is subject to an ongoing investigation (and these were destroyed while they were under investigation) it is assumed that the evidence would have been damning (the destruction of evidence is itself a crime on top of it all).
Using a private email account that goes through a private server while working for the Federal government isn't a crime, and won't ever be. Even arranging some official business on private accounts isn't, so long as those emails are later entered into the record (oh, and FYI this is ANOTHER failing of Clinton's, we know she used her private emails for doing official business BUT never entered them into public records*). The only thing that you can't use private email for is the transmission of classified information, to transmit that you MUST use a properly cleared government email service (which, FYI, isn't even the regular government email, basic government emails are only cleared for Sensitive But Unclassified date, Classified Data MUST go through special email on a special secured network).
The media completely mishandled how they reported on the Clinton email server scandal. They made it about having a private server, but it wasn't really about that, it was about the mishandling of classified information. And they can't even say that can't report well on that issue, because they've managed to make it clear about those issues when it comes to Trump even though what Trump did is considerably less of a security risk than what Clinton did.
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* FYI this isn't an onerous processes. Simply including your government email on the CC line of an email you're sending from your private account as official business is enough to fulfil this requirement. She didn't even do that, which is pretty much prima facta evidence that she was attempting to CONCEAL what she was doing from public view and information requests.