Their is several things going on, but the most important one is that political models are changing. The old rules of how politics are played are no longer working in a post Trump world, and no one really knows what the new rules are making it practically impossible to predict what is going on. So, polls are less reliable, because they don't really know what to as or to who and everyone has their own idea about what the right methodology is. That is on top of the problem that polls are generally getting harder and harder to do as scams after scam has made people less likely to respond to random people asking them questions.
In a few more cycles it might settle down again as different methodologies are proven effective and others are not. The only other worry is that there has been a rise in the use of propaganda polls, ones that use intentionally skewed methodology so as to get a specific result in order to sway public opinion. It will not be the first time we have seen that tactic used in American politics, but in the past it has been deemed counter-productive to long term health of a news source. Now it seems that some large news sources are more interested in politics than profit. That changes the calculations.
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