From communications prof Mark Balnaves at MercatorNet:
Thirty years ago, pollsters could use landline phone numbers to amass a statistically representative group of people. For example, reverse telephone directories ensured that pollsters would be fairly confident on their sampling frame and the identity of participants.
The shift to mobile phones and internet polls have meant that pollsters are now faced with a more fluid situation. Not as many people are willing and able to take calls from pollsters on their home phone – if they even have one – which can affect the quality of the data. And after all, pollsters can only work with the data they have.
It might appear that with the rise of the internet, there are now easier and more reliable ways to divine the citizen mind. But what is gained from “rich data” (what people say in detail) has to be balanced against the loss of a reliable sampling frame – not everyone has access to the internet or the time to fill in internet polls. More importantly, perhaps, unlike the earlier conventions polling is no longer deliberative and participatory.More.
Reality check: Another reason, worthy of a book probably, is that most polls in such a contested election are efforts to sway the outcome or to reassure the pollster’s constituency that it is worth continuing to fight. They serve about the same purpose as speeches to the troops and should be considered to be about as reliable as to outcome.
See also: Why not to trust 2016 presidential polls: Many issues prevented pollsters from accurately building their models.