I know this is a caricature, but something about the Obama years and many Democrats... I just don't get it. My role as an asshole on the internet is to agitate to make things better, to point out that things sorta suck (and, really, they do, good things happened over those 8 years, but so did bad things), and to encourage the Democrats (or whoever) to respond to that fact to try to make things better (or at least pretend they want to because that's good politics!).
There isn't one specific defining reason for the 2016 loss, especially if you limit that question to "how did Hillary Clinton fail to trip over the electoral college finish line and Donald Trump did instead." But, roughly, "four more years!" wasn't good enough to make her win inevitably against Donald fucking Trump and there are reasons for that. That's more Obama's fault that Hillary Clinton's, but even if you think Obama gave us the best possible world he could given the constraints he faced, he certainly didn't give us the best possible world we can hope for. That's Jon Chait's view (basically), but it sure as hell isn't mine.
Avocado Toast. I blame the Avocado Toast.