The problem is that people on the left don't vote, and when they vote they don't vote for the Democrats. The more extreme side of the progressive party never votes democrat, so why should dems every listen to them? Their best chance at getting elected is to try to appeal to moderates who have been disenfranchised by the insanity that is the Republican party.

By contrast, all the MAGAtards, gun nuts, religious zealots, nazis, racists all happily fell in line under the Republican banner even when the party line was not quite as extreme because they knew they could change from within far more effectively than constantly trying to hold their votes hostage.