Following up on Warren Kinsella.
1. Sometimes, questions about voting intentions are asked on non-political polls. For instance, you could be asked to do a poll on banking products or coffee shops, and then at the end, the pollster asks the respondent who they will vote for. The sample is therefore very skewed.
2. People filter out unwanted calls. Fewer and fewer people are picking up their phones. Many use call display or screeners.
3. More people are on cell phones and polling companies won't perform surveys on cellphones unless the customer says it's okay. As you can imagine, the vast majority would not want to waste precious minutes on a survey.