Dems do have a constituency (boomers who own their home and are pretty racist but not extremely racist), but more importantly they have legitimacy in the eyes of the people. Largely due to propaganda, but counteracting that propaganda machine is difficult, if not impossible. Bernie brought in a decent amount of money from normal people, but you have to consider that between them, the DNC and GOP control all of the mainstream media in the country, and have enough capital to do a decent job at controlling social media as well. Remember the coverage of Bernie during the primaries, and consider that it would just get even more malicious if he was running as a third party.