Because they had no other choice in the last election, that's why. I know you like to think it was because Trump won them over with his MAGA crap, but you're wrong. Sorry for not saying what you wanted to hear.
The simple fact is this: Most Democrat voters aren't far-left socialist nut jobs, most of them are kind-of-left with kind-of-right sympathies. They want stuff like affordable health care coverage and higher paying jobs for the average working schmuck, but they don't want to resurrect the friggin' Soviet Empire to get it.
That's where the Democratic Party screwed themselves last time out. The people wanted something at least slightly reasonable, and the Dems offered them the unicorns-and-rainbows version of the Karl Marx playbook. Everyone who is old enough to legally drink knows that crap isn't gonna work, so they voted for the only option they had.
If the Dems figure out that the center is the real high ground in the upcoming campaign, they'll bury Trump in a landslide. But if they keep doubling down on that Sanders-Lenin bull shift, they might as well not even show up.