But as things stand the Dems have done just about everything to crush challenges from the left (e.g. contesting Green candidates on state ballots), and to funnel every people's movement (including civil rights and labor movements) into their election machine only to backstab their supporters
And they use the credible threat of GOP's ruthlessness to demand we fall in line into their popular front. This worked for decades but as all the major civil rights gains are eroded the legitimacy of this coalition and credibility of its premise - they're the best defense against GOP - are waning.
Despite this there's still a huge gap between recognizing both Dems and GOP as The Class Enemy and actually having a viable alternative ready to go. Until we build one we do not have the same agency the Democratic Party does.