Bernie Sanders shares a number of characteristics with Ben Carson, Rand Paul, and Ted Cruz
His ideas, from an academic and economic standpoint are archaic and feel good. Anti Immigration, anti Free Trade, Transaction tax, etc. Not to mention, hugely increased taxes on everyone is not a turn on for many people. Similarly Ben Carson supports feel good but somewhat impractical things, like a true flat tax, ending political correctness, etc.
Like Dr. Carson, he has no clue about how foreign policy and terrorism work, and he deflects anything he doesn't understand to his 1% stump speech. This is not good for a candidate, especially one facing Hillary Clinton.
He has limited appeal outside middle class white progressives. Even when he talks about race and gender issues he always finds a way to tie it back to his 1% stump speech, which sends the message that everyone is oppressed, which middle class to upper class white kids like to hear (see Rand Paul). This, and the fact that he, like many on the news, advertises that he 'participated' in the civil rights movement to show he cares about minorities. His focus also lies with giving everyone, regardless of class and race free secondary education in the name of income inequality, instead of improving overcrowded schools in the ghettos and giving more scholarships to those who truly need it. As someone who went through the college process 3 years ago and is relatively affluent, I can tell you that in nearly every state there is a school, state or private that has sub $15000 a year base tuition. If you have solid grades and whatnot, you can get good scholarship at a T3 school. Unless you have zero support at a private out of state school, and you go into a major with little job opportunities (there are still jobs in Education, Computer Science, etc.) you shouldn't rack up 200+k in debt in four years.
He is a progressive white male Northeastern candidate facing Hillary Clinton and likely a minority from the GOP. This alone is a hinderance. Not to mention, he won't get squat done in Congress , much less the GE id he doesn't compromise his positions.
Even Elizabeth Warren would fare better than this dude.