Drug the masses with false reality
The issue is that a LOT of US polls were WILFULLY incorrect. For example, many polls on realclearpolitics listed democrat samples much higher than the republican samples; and it was mostly in these polls that Hillary was ahead. Looking closer at the polls, you could tell that if a heavily weighted Dem poll still only had Hillary up by 1 or 2 points, she was in serious trouble.
Call me a tinfoil hat wearer, but I do not for a moment question the possibility of the US media trying to influence the outcome of the election because Hillary represented the safety of the status quo for domestic policy (despite the danger of a third world war with international policy). I cannot think of another reasonable explanation for consistently oversampling democrats in most of their polls other than an attempt to demoralise the opposition or convince readers of what the "right" opinion to have was.