The problem with polling is that they can only call landlines and the mix of people who they are able to reach that way no longer resembles the general population. What sort of people still even have a landline? So the pollsters have to compensate for this using polling models. They ask questions about demographics in addition how they plan to vote, put each datapoint into one of numerous bins, and then apply a coefficient to each bin to guesstimate the real total support for each candidate. The problem is that the models are only wild guesses. No one really knows the turnout percentage will be for each demographic slice of the population. To make things worse, Trump supporters are notoriously uninterested in sharing their true opinions. And pollsters and their clients have their own biases that they unintentionally (and intentionally) bake into their polling models.
In short: the polls are broken.