With political polling, as with credit ratings, the person or organization buying the service tends to get the result they want, regardless of claims of impartiality. From the Zman at theburningplatform.com:
It is becoming popular on the Right to dismiss polling as nothing more than another part of the endless gaslighting from our rulers. Like the news, it is assumed that the polls are fake, at least regarding Donald Trump. For sure, there are a lot of bogus polls and even more bogus poll stories. Like everything else in this age, the quality of polling has steadily declined. That said, there may be some other issues beyond deceit that are undermining the utility of political polling.
One obvious problem is the demand side. The media has always liked polling because it is cheap content. It allows them to discuss the results of an event, like a presidential election, for months before it actually happens. Polling stories are just a way to pretend the election already happened. Then as the polls change, and they always do, the media can talk about those changes. That also opens the door for so-called experts, who can provide “expert” commentary on the polls.
Of course, supply naturally follows demand, so Gallup was followed by other polling outfits supplying opinion surveys to the media. The explosion of media over the last thirty years has caused an explosion in demand for polling. One reason we have so much bad polling now is the same reason we have fake news. There is only so much good quality material, so the void is filled with low-quality material. Fake news has replaced real news and fake polling is crowding out good polling.