Time and time again when progressive parties try to position themselves as a vote against conservatism they lose. The voters who stayed home aren't to blame, the blame falls on the party failing to sell a vision of the future to get them out to the polls, which is their one job!
Being constantly told that better things aren't possible and that we have to be "practical" and vote for the lesser evil has just convinced people to check out entirely, stop engaging, and stop voting.