Like 99.9999% of liberals I'm not actually in charge of the Democratic party or their broader strategy. Come election season I can choose
A) To vote against republicans in the general election
B) To vote for more liberal parties in primaries
That is basically it. The effects of electing Republicans is often substantial risk to our nation and to myself personally so I cannot afford to score political points by voting third party nor is there good reason to believe this would be effective. The historical fact for 40 years has been that winners on the left side of the aisle had to appear attractive to moderates and liberals which limited how far left it was useful to get. Not to mention most far left positions are bad for the rich and corporations at least in the short run which leads to money drying up and the moderates who get said funding and more support from fellow moderates winning.
Basically folks like you are threatening other folks with losing if they don't execute a losing strategy. This is why it didn't happen because you weren't actually offering a path towards victory. Maybe after Trump completely ass fucks our country and probably impoverishes it for a generation we will actually have more will for real change but I wouldn't have chosen to sacrifice my family to achieve it.