Vox has a great, detailed interview with Democratic-aligned polling expert David Shor. Shor’s theory: Trump voters aren’t “shy” and lying about their voting intentions; they’re just not answering polls at all. People who answer polls tend to have high levels of civic engagement and trust, and in the Trump-and-COVID era there’s a much bigger gap between the parties on those attributes than there usually is.
After 2016, many pollsters tried to do a better job of “weighting” their surveys to match the demographics of the general population, particularly in regard to education. (If some demographic group is 20 percent of the general population but only 10 percent of survey respondents, the folks from that group will get higher weights so their answers have more sway.) But demographics don’t fully address this issue. If, say, high-school-educated white Biden voters are more likely to answer surveys than high-school-educated white Trump voters, it won’t do to rejigger the weights on high-school educated whites in general.