You forget that he was leading in the race early, and he had a high approval rating among Democrats. He was poised to win the nomination until South Carolina, where an intense campaign by the DNC and other centrists convinced Black voters to back Biden by a landslide. After that the other candidates conspired to support Biden and undercut Bernie for Super Tuesday, and it was over. Bernie was railroaded by the DNC machine - the same as was done in 2016 to anoint Hillary. To say that he was not popular is not true at all.