Why were the polls wrong about the result?

I recommend this article on the Financial Times today: https://www.ft.com/content/8de13154-a677-11e6-8898-79a99e2a4de6 (Trump victory: corporate winners and losers). Their argument may not be perfectly precise, but definitely provides us with some details about what Trump's possible future policies could be according to the content of his previous election campaign speeches. As I have expressed my expectation about how Trump will lead the US, I am not going to talk about this issue today.

Many people feel that the victory of Donald Trump is like Déjà vu as Brexit had a similar situation before. The media find that their previous polls were quite wrong about the final results. Last time they were wrong about the Brexit, this time they were wrong about the US election. Why did these polls fail to predict the election outcomes?

The first major problem is that these polls failed to cover a fair sample, the samples they select are generally biased due to the ways they used to reach their interviewees. Moreover, some voters are only active on the election, and the media is not able to reach them any other time. The second problem is some people are not willing to speak about their true preferences as their preferences are considered to be different from the social mainstream opinion. This could cause the results the media gets are biased towards their favor. The third problem is that when the voters feel they are the minority, they have stronger incentives to vote than those who feel they are the majority.

Therefore, when it comes to a situation when the mainstream media has a very strong bias, the polls can be conducted to be biased towards their views despite their willingness to conduct a fair poll. This means when the mainstream media has a strong bias towards a particular result, the polls they conduct are very likely to be biased towards that result; therefore, the probability of get a wrong prediction increases.