...(although the first may piss off some here):
8.) Joe is not a Clinton. Seriously, most of the people on the left that I know of who were really unhappy with the outcome of the 2016 primary race were those who had worked to get Bill Clinton elected, only to see him eagerly (and, some would say, unnecessarily) "compromise" in ways that enacted tons of Republican "wish list" legislation (NAFTA, severe restrictions on AFDC, DOMA, banking deregulation, the "superpredator" crime bill, etc., etc.) without really ever getting anything in return. While it's possible that Hillary Clinton could have charted a more progressive course than her husband, the distrust ran pretty deep, and she frankly didn't help dissipate it by mockingly dismissing any progressive proposal as "free ponies!" and the like. But, even among those who didn't have an ideological dislike of Clintonism, there was a definite "Clinton fatigue" from those years -- if Bill was in some sense a "teflon president" who had nothing stick to him, it could be said that what didn't stick to him stuck to those around him, rendering both Al Gore and Hillary into "velcro candidates."
and, of course
9.) As others have pointed out above, polling organizations re-balanced their demographic allotments in the wake of 2016 to correct for what caused them to be off that year. In general, that meant assuming that less of the white portion of the electorate had a college degree, and thus increasing the weight to non-college-educated whites. In other words, the polls we see today have the 2016 correction already baked into them.