Well, frankly, as Stranger On A Train notes, the polling companies have really struggled with the decline in land-line phones, and with ensuring that newer methodologies (cell phones, online surveys) are representative. (Non-political market research, which is my field, has suffered from the same challenges.) Getting a representative, accurate sample was certainly an issue even back when 95+% of households had a land line (and were willing to answer their phones), but it’s a much, much bigger issue now.
Gallup, a research and consulting company whose name has been synonymous with political polls for decades, used their political polls as a way to build recognition of their company’s name, so that they could sell other sorts of polling and surveying to clients. But, they’ve been stepping back from political polling for several years, because of the rapid increase in the difficulty of being able to run representative, accurate polls.
And, as 538 will frequently note, there are a lot of political polls out there, and quite a few of them suffer from poor quality (and some of them are likely to be intentionally biased).