Why are public opinion polls failing?

Generally speaking- and opinion polls of opinion pollsters agree – the failure of public opinion polls is entirely due to the people polled.

US President George W Bush put it accurately, if somewhat rather clumsily, when he said:

It's no exaggeration to say that the undecided could go one way or the other.

Quite frankly, the people aren't being fair to public opinion pollsters – mind you, that is just my opinion.

The American Association of Public Opinion Research (AAPOR) has released its autopsy of the 2016 US presidential election and. given the fact that AAPOR is the industry association for public opinion polling organisations, it was unlikely to be severely condemnatory of the self-evident fact that their member organisations very largely – in fact, overwhelmingly – got the actual outcome catastrophically wrong. Hillary Clinton was so bedazzled by these pre-election polls that she didn't give a nanosecond's thought to preparing a concession speech on election night last November.

The Executive Summary of the AAPOR report begins:

The 2016 presidential was a jarring event for polling in the United States. Pre-election polls fuelled high-profile predictions that Hilary Clinton's likelihood of winning the presidency was about 90 percent, with estimates ranging from 71 to over 99 percent.

Having got that self-evident truth off its corporate chest, the AAPOR report then tried to defend its own member organisations. Frankly many of these defences are specious while many are demonstrably absurd but overall the defence is that lots of people either fibbed to the pollsters or just changed their mind on election day or – mostly probably - both.

The report states:

National polls were generally correct and accurate by historical standards. National polls were the most accurate in estimating the popular vote since 1936. Collectively, they indicated that Clinton had about a 3 percentage point lead, and they were basically correct; she ultimately won the popular vote by 2 percentage points. Furthermore, the strong performance of national polls did not, as some have suggested, result from two large errors canceling under-estimation of Trump support in heavily working class white states and over-estimation of his support in liberal-leading states with sizable Hispanic population.

If anybody should be acutely aware of the US Electoral College system of electing their President you would think American polling organisations would be which, of course, makes national polling figures almost meaningless and, in this case, deceptive.

It continues, "State-level polls showed a competitive, uncertain contest" yet it goes on to say, in apparent denial of this uncertain contest conclusion that "Clinton appeared to have a slim advantage". Does a "slim advantage" actually mean an "uncertain contest"?