I already gave an answer earlier in the thread that is true for the two similar polling misses in the UK and my best guess for what happened in the US: differential turnout. Trump supporters voted with higher probability than they had historically (or Clinton supporters voted with lower probability, which seems more plausible looking at the turnout figures). So even if people's preferences for president were completely honest and recorded correctly, the projected "likely voter" poll results were wrong.

This is known to have happened in Britain, because there was a very thorough postmortem by the pollsters' professional body after the 2015 miss.