Warren’s struggles with Medicare for All played against a backdrop of other authenticity issues, most famously her purported Native American heritage. Bernie has no such issues, in fact the opposite.
No one can match his socialist cred, built up over decades. Only Bernie honeymooned in the Soviet Union. Only Bernie has videos out there praising Castro. Only Bernie, back in the day, could try the patience of other socialists by constantly talking about socialism. Only Bernie has never formally joined the Democratic Party. Only Bernie took on the party establishment single-handedly in 2016 and almost won.
Sanders says things that are fantastical and untrue all the time, but out of sincere belief in his program. He’s shifted on issues like guns and immigration over the years, but still gives the impression more than any other major politician of simply being incapable of going out and mouthing poll-tested bromides he doesn’t believe.
His grassroots army and small-donor fundraising base gives him an independence from traditional Democratic donors and their world that no other major candidate has.