Suppose your party’s candidate is going to lose the election in a landslide. What follows? A resounding mandate for the other side. Lots of effective policy from the office of the widely supported President.
Why just sit and take it? One way to be a spoiler is to delegitimize the other candidate. I believe the FBI passed on Clinton to avoid stepping into a political turd. First, FBI employees would be vilified and attacked in the mainstream media without mercy. Second, the resulting Trump victory would be viewed by many as not an endorsement of his policies but as the inevitable result of an undemocratic criminal prosecution. Even if you think Hillary personally committed crimes, you probably do not want all the people who believe in her positions disenfranchised. If there were time for the Democratic Party to put up a replacement candidate you would be perfectly happy with the contest, but there is not.
Now of course you can say she should have thought of that before she betrayed her supporters. In a way, however, people who believe in her politics shouldn’t pay for supporting the only representative of their views on offer.
Much more important is that disqualifying her candidacy delegitimizes her opponent’s victory. Do you want the President selected by the discretionary decision of an FBI director, even if you agree with his conclusion? What of the people who do not agree with his decision? You cannot tell them they lost fair and square in a vote, except with a very sophisticated argument with many logical steps (don’t run criminals and you should know if your candidate is a criminal). Surely a victory at the ballot box without indictment of your opponent is the better foundation for a successful Presidency.
So now it appears criminal investigations of the email server, the foundation, the Dept of State grind onwards towards what? Do these Democrat controlled bureaus really want to bring charges against their candidate three months before the election with no possibility of resolution before the public is asked to decide? Are they impartial dispensers of justice or strategists trying to at least spoil the other side’s victory?