... Lacking authenticity?
Yes.
Bernie's campaign has had unprecedented grassroots support. It's reached 2 million individual contributions faster than any US presidential candidate in history. Both Hillary and Bernie have raised more in donations than any other candidate with Bernie having raised a few million short of Hillary. However, he has accepted no Super PAC support to achieve this.
This is what makes Sander's a real threat to republicans.
Let's also not forget that Bernie is beating any republican in a hypothetical match up.
It is slowly looking like Sanders will be the next president. He does have support across party lines, a lot of support with independents, and he HAS risen as a result of a grassroots movement. He was polling like O'Malley six months ago, now he may win some of the primaries. And he has no ugly baggage.
When you step back and weigh things without regard to party you start getting that sense, small but building very slowly, that this guy is going to win the candidacy and the presidency.
The Republicans have been throwing mud at Hillary for 4 straight years thinking she would be the nominee. Sanders is the one that will benefit from it. trump can't win a general. He has no black, Hispanic, or women's vote, he's done if he's the Gopp candidate.