The media was initially skeptical of trump as a serious candidate because he didn't have the background which was presumed necessary. Prior to Trump every single presidential candidate had either had elected experience of some sort, had served in a cabinet-level position, or had been a general in the armed forces. Donald Trump and deducting the first Presidential nominee, not to mention winner, who had never served in government in any capacity. Trump and Eva never run for anything before or seriously been involved in politics. This made his candidacy highly unlikely. Trump also seems unlikely to win because he kept making a lot of gas that generally derail candidates campaigns. For example, he denigrated the war service of John McCain.
Once Trump became the nominee, his like that the beating Clinton was considered extremely low because it was extremely low. Every single pole showed him trailing Clinton, Often by large margins. I know that Trump supporters like to be dismissive of the poles but they were actually accurate on the national level. The final set of them show Clinton beating Trump by about 2% and that's about what she won by in the popular vote. So we've got to assume that the other polls were accurate as well. And if they were, then the media was correct in assuming that Trump was going to lose. As is, Trump barely won the election. All that separated him from A Loss was about 80,000 votes in three states. That's microscopic in an election where over a hundred and twenty million ballots were cast. And this victory of his, as Slim as it was, only came after an October surprise significantly depressed Clinton's poll numbers. Anyone looking at the race in October would have been completely accurate and saying that Trump was going to lose.
Ironically, I think that the reason that the media underestimated Trump is the exact opposite of why you think it is. The Trump supporter argument is that the media did not take Trump seriously. In reality, I think that they discredited his chances because they took the Republican electorate has more serious than it was. The media, contrary to what many Republicans allege, wanted to be fair to Republican voters and did not think that Trump's brand of racism and bigotry, backed up with no confirm experience, would be appealing to them. The media assume that Republican primary voters would be serious in selecting a candidate and would dismiss Trump because of his lack of experience in many idiotic statements. They furthermore assume that Republican voters in the general election would be lukewarm a trumpet best and wood be reluctant to support someone who was obviously so unprepared for the presidency. Ironically, if they had treated Republicans less seriously, and been more willing to embrace the idea that Republicans value racism and bigotry, then they might have rated Trump's chances more highly