Because the lost the last elections. But at the pace Trump is taking over the all the agencies and institutions of the US, two things may happen: No more elections, from now just a dictator ... OR the people get so pissed of at Trump destroying the US that they eventually vote the Democrats again no matter what the candidate or the messages.

The Democrats have to recover the wider opinion and public base, and it may be easier after 4 years of the current POTUS.