Nothing to do with 20 years of previous investigations; just the four years of her being Secretary of State and having something quite valuable to sell.  The email scandal was a indication of a much larger crime, one the "Justice" Department refused to prosecute.  If you were a prosecutor, would you take on the next POTUS?  Especially one as vindictive as Hillary Clinton?  I was in the military and used to run an organization dealing with communications and computer networks in a most highly classified environment.  The "decades of investigations" argument has been overused and found invalid.  Just the "Uranium One" scandal is so huge it could result in life sentences alone.  Bradley Manning was just one of many examples of what happens to people who screw around with classified communications.
I love how the Clinton Foundation reporting has completely dried up now that they have nothing to sell.  The foundation itself is completely failing.