Bernie Sanders is new to the Democratic Party. In the past, he has run as an Independent Socialist -- and been very successful in his small state of Vermont. But now that he is running for president, he realizes that an Independent has little chance to win -- so he picked the Democrats as the party closest to his own views.
And he has been able to get a significant following among the more progressive Democrats -- those for whom the word "socialist" is not a scary thing. And while I support Hillary Clinton, I agree with my progressive brothers and sisters who see socialism as a good thing instead of a scary one.
But like it or not, most Americans are not as open-minded about socialism. The right in this country has demonized that word, and convinced a lot of people that socialism is just another word for communism. It's not even close to being true, but that is the general perception of many Americans.
Progressive Sanders supporters overlook this, and remain convinced that an open and avowed socialist (which Bernie Sanders is) could win a national election. I wish that was true, but I just can't bring myself to believe it. There is still too much of a negative connotation attached to the "S" word, and you can bet the Republicans would make it a primary election issue. I am convinced that would be a death knell to Bernie's chances of being elected.
Just look at the chart below. It was made from a Gallup Poll done between June 2nd and 7th of this year. Note that people would be more willing to vote for a muslim (60%) or an atheist (58%) than vote for an admitted socialist (47%) -- tow of the most demonized groups in the country. In fact, a socialist candidate is the only type of candidate with less than majority support.
This is not going to be an easy election to win for Democrats -- and it would be crazy to nominate an admitted and open socialist as the Democratic candidate. I like Bernie, a lot -- but his history of running as a socialist would scare to many voters off (and they would vote for a Republican -- even if they had to hold their nose to do it).