Trump's message resonates strongly with white racist voters. This isn't a matter of Democrats doing something wrong. They are doing something right by appealing to the majority with a broader perspective on society, diversity and cooperation. You Cult Of Sanders idiots are part of the problem by insisting on purity of liberal ideology when most of the country is not ready and has not bought into those policies.
Frankly, what Bernie Sanders and you liberal idealists consider finished policy is laughably unready for implementation. People are quite right to reject it. For example, Bernie's health care bill which would force everybody into Medicare regardless if they would prefer to keep their current coverage. He released that bill last year without a price tag, no funding source and no plan for how to manage the growth of Medicare.
Trump is a powerful voice for the roughly 40% of people who are white and lean toward racism. It's going to be a hard fought campaign season and the DCCC quite reasonably is selecting candidates who are good fits for their district. This means they wouldn't choose a California liberal to run in a district where Trump won and especially not in districts where they have to unseat a Republican.
I guess you thought it would be easy. You never were very good at reasoning things out.