Serious thinkers set to work, and produced a long shelf of books answering this question. Their answers tended to rely on similar themes. First, Democrats lose because they are too intelligent. Their arguments are too complicated for American voters. Second, Democrats lose because they are too tolerant. They refuse to cater to racism and hatred. Finally, Democrats lose because they are not good at the dark art of politics. Republicans, though they are knuckle-dragging simpletons when it comes to policy, are devilishly clever when it comes to electioneering. They have brilliant political consultants like Lee Atwater and Karl Rove, who frame issues so fiendishly, they can fool the American people into voting against their own best interests.
To say that the Democrats are condescending is woefully inadequate. Remember the book "What's the matter with Kansas?" Why didn't the writer do another book "What's the matter with the people who live in D.C.?"
Further proof that the narrative is more important than the facts.