

A couple reasons.
Bernie and the Democrats are going to align 90% of the time (just like with Manchin on the other end of the spectrum). Basically, the other party is anti-America. So being pro-America makes you a de facto Democrat.
Since we don’t have Ranked Choice Voting or any other system that is conducive to multiple parties, primaries allow directionally aligned candidates to decide who is most popular to avoid splitting the vote. This isn’t necessary if a candidate has enough support for it to not matter. (Bernie is an independent in congress, but he ran as a Democrat in his presidential runs.)
Just like the polls that show McDonald’s, Taco Bell, or Chic fil a as America’s favorite restaurant in X state. It doesn’t take too many idiots to outweigh everyone else.