The poll models underestimated turnout of 'white non-college' voters (the white working class and rural whites) and overestimated turnout of urban PoC, which is why they were off by several points. Hillary didn't campaign in those states because both public and private polls said she was way ahead, and Comey's actions 11 days before the election sealed the deal (by shifting the undecideds heavily to Trump).

Alot of people on Twitter and Youtube noted that our convention was too touchy feely, and some of the musical performances were mocked by young people (especially the 'For What it's Worth' performance). Like it or not, some people are moved by 'fire and brimstone' and want to see some level of toughness and straight talk about policies.

I've never liked Sanders and thought he was vindictive in 2016. But I think this time he is operating in good faith and is just telling us what he is seeing amongst the demographics he handily won. Nevada concerns me as there have been several reports that Biden is underperforming with Latinos and youth as he did in the primaries. Bernie crushed in Nevada and we need his help, also with other swing states that could be decided by Latinos (especially young Latinos).

TLDR; we need to ignore the national polls and pay attention to state level and demographic trends, which are showing some weaknesses with certain groups, as we don't want to be 'surprised' and lose states we think we're going to win.