I think the proliferation of cell phones over landlines have presented a problem for modern pollsters. Many cell phone users are reluctant to answer the phone if caller ID shows it to be an unfamiliar number. But most people won't think twice about answering their landline, regardless of caller ID. This must be a factor, somehow.
There is also the polling questions. Pollsters decide to count a poll by determining if this is a likely voter, usually by asking "Did you vote in the last election" and "Are you planning on voting in this election"? A likely voter will respond Yes to both questions. But in this election, you had people answering No and Yes, respectively, to those questions.
In my own case, I did get an automated call to my landline "We'd like your views on the Presidential election". But after two minutes of asking me questions on issues, I became impatient and hung up, never being asked the Trump vs. Clinton question.
Lastly there is privacy. "Your views will be held in confidence" is Zukerberg-laughable when they already have your phone number.