This would be true if Democrats didn't actively surpress and ignore younger and progressive voters and candidates, instead focusing on a non-existent class of swing voters.
If a candidate goes out of their way to not represent you, derides and ignores your needs, why would you vote for them?
The Democratic party is broken by corporate greed and has been since Citizens United. People are wise to that reality and have no interest in participating in a system that doesn't protect them, fight for their rights or show any real interest in progress.