Understand the players here. The State Department is run by John Kerry, and has been engaged for months now in a foot-dragging exercise of "reviewing" and releasing (only because a court ordered them to) the Hillary Clinton e-mails they absolutely had to - withholding, of course, those containing top secret information that Hillary still tries to say do not exist. The State Department would like nothing more than to bury this whole thing and pretend it was all a big nothingburger in the first place.
The FBI is run by James Comey, who was first appointed by George W. Bush, and consists of career law enforcement professionals. Their bosses are political - led by Obama-appointed Attorney General Loretta Lynch - but they conduct their investigations in a serious and unbiased manner. So when the FBI sees the effect the State Department's slow jam is having on its own investigation, it tells Foggy Bottom one thing: Stop: