I think they were so overconfident that they slacked off a bit too much.
This Link is an interesting take on how jubilant the Clintons were just hours before the election was called for Trump.
The guy talking is pretty boring, go to about 36 seconds to see the Clintons dancing around with absolute knowledge that they were going to steal the election.
If you have time, listen to the whole video.
There is an interesting statement that the narrator attributes to their over confidence.
He says that this is right at the time they got a call from "daddy Bush" that the stealing of the election would work.
Bill does indeed look like a giddy school girl the way he is jumping around with unfettered glee.
If theyy were overconfident thay would not have cancelled the fireworks two days before the election. Romney likewise cancelled his entertainment prior to the election when his internal polling told him he was going to lose.