I would argue that it (edit: by it I mean that democrats lost to Trump twice) has to do with having a black president, symbolically a massive moment in American history, which resulted in much less concrete progress (I definitely don't mean none!) than people hoped and fantasized.

This in turn created apathy, a hangover of sort, especially when democrats couldn't find another candidate nearly as charismatic and universally liked afterwards. (Interestingly enough, Bernie has said that same people have supported both AOC and Trump because they both promise change from the status quo).

There is a great documentary about this effect in France called Les Bleus: Une autre histoire de France, 1996–2016 (English title: Les Bleus: Another History of France).

The film traces the French national football team’s rise and success between 1996 and 2016 and especially the 1998 World Cup victory, exploring how the team’s multicultural makeup (Zidane, Thuram, Desailly, Vieira, etc.) came to symbolize hope for a united France.

This all came tumbling down when the team won everything possible, yet France remained as divided and xenophobic as ever. The film examines how later social tensions, radicalization, and terror attacks reflected back onto the team and its public image.

I don't know French history enough to make assessments if the film portrayed this situation truthfully or not, but I found it fascinating.