It is easy to understand. The people who the Republican party appeals to are unlikely to vote Democratic, the Democratic Party by trying to appeal to them it is losing its base while not winning any Republican voters. The people who are likely to vote Democratic are more idealistic and will hold the Democratic Party to a high standard. The promised "push to the left" never came, and so the people are looking at two right wing candidates, the Republicans already got their guy in Trump and anyone even slightly left of center has no one to represent them.