My take as a former Democrat voter (Kerry '04, Obama in '08):
It's a combination of Trump himself - the give no fucks, celebrity/president cult of personality type who represents a backlash to ultra Political Correctness and this sort of modern strain of Neo-Puritanism that is running rampant in America (and yet funny enough I think most Americans, even many liberals, are more than sick of)
..coupled with - zero enthusiasm or real substance by most of the Dem candidates. Many are either the lame, old breed of Dems like Biden who's basically a centrist neolib/Corporatist types with no new or exciting ideas, or the nuttier Bernie/Warren new-school types who may have some interesting ideas but have something of a limited appeal with how far left they lean. Most of them seem to be running on a campaign of negativity in one way or another, with a focus on Trump hysteria rather than focusing on actual ideas to move America forward or how to cater to most Americans.
Trump on the other hand DOES present this message (whether true, bullshit, or somewhere in between) and people are drawn to that.
You're not going to garner much support placing so much emphasis on negativity and essentially what amounts to "Just vote for me so this guy who's worse than me doesn't win again!" WHY should we vote for you, specifically? What will you do better as president? I fail to see most of these candidates answering these questions.
So I feel it's just like you've got a couple corporatist neolib Hilary types at the top (Biden and Harris, maybe Booker? namely) who don't generate much excitement, and in the far left camp or newer breed "AOC" school of Democrats, you've got a highly crowded arena of candidates cutting into eachother's bases and pandering to the same audience. And all in this camp seem to be trying to "out liberal" the next guy, as many seem to be adopting increasingly odd and extremist views and policies that are just never going to have a massive appeal in the US.
I DO think Bernie seems to draw a similar amount of enthusiasm as Trump (he's sort of the anti-establishment left candidate toTrump's anti-establishment RIGHT candidate), but the media oddly seems to not care much for him, so he's largely ignored or at least others are given favoritism over him.