Ultimately, Trump's DOJ could decide to file charges, so far, they haven't.  If they wait too much longer, it will look like they are trying to distract from his obstruction of justice problems.  This isn't hard, he swore that he would put Hillary in jail on day 1, we all knew that was BS, but had he followed through, Sessions could have pushed more publicly in the very early days of his administration to re-open the investigation.  While I think she probably needed to be charged, at this point, the optics will make it look like he is trying to distract from his own problems.