Up until the results started to come in, it had been widely believed that Hillary Clinton was going to have won the Presidential election, regardless of whether you supported her or not, and this was reflected even in the polls, only for Donald Trump to have won the electoral college. There have been a few theories going around suggesting why the polls could have made such a glaring mistake.
The simplest explanation is that Trump voters were simply unaccounted for
A good chunk of the theories that have been proposed give the pragmatic explanation that supporters of Donald Trump who ended up voting for him were simply underrepresented, but the exact nature behind this has been debated.
On one hand, it is entirely possible that many of the people who ended up voting for Trump were supporters who were simply keeping their true views under-wrapped during the whole election process. With support for Clinton and disdain for Trump being very vocal in American media up until the election results, it can be argued that this theory has some validity to it. Alternatively, it has been theorized that Trump voters actively avoided polls, possibly due to fear of bias or manipulation.
On the other hand, it could be argued that it may have been the fault of those running the polls to properly reflect those voting for Trump. These pollsters could have relied too much on demographics to focus on areas that already seemed to have supported Clinton, and ignoring her opposition.
The theories get stranger and stranger
Now, even towards the very end, people on both sides of the fence made the expression “anything could happen at this point” a trending catchphrase, and it might hold some validity here. As strange as it may seem, it is entirely possible, however unusual, that American voters actually did change their minds at the last second. This theory is helped by the fact that Trump won over Pennsylvania, a state that seemed to have been in Clinton’s corner until the end.
Alternatively, as strange as it sounds, the media’s advocating people voting this election may have backfired on Clinton, even if it seemed that most of it was directed at her. Between Emma Watson issuing a public statement pleading with the American people to vote for "Will and Grace" coming back from the dead to ask the same, every prominent voice in the media tried to make their voice on this issue heard. That said, the people who were inspired to vote this time around decided to support Trump, and since they may not have been people who voted before, they were not put into consideration during the original polls. In other words, they were put in the “not likely to vote” column. #Election 2016