I agree neoliberals are a very big part of the problem (and giving them the finger was one of Trump's trump cards, pardon the pun), but I don't think it stops there.

Another aspect, which applies to neoliberals and many other groups in the Democratic coalition, is the seeming abandonment the idea of representing voters (in a district) in favor of persuading/lecturing them. That really appeals to the true-believer partisans (who are getting represented), but alienates everyone else.

I'm pretty sure the Democrats would have at least 5 more Senate seats if they consistently ran candidates that focused on representing the state they're running in as a whole (and continued to do that after the election was over).