This is the real reason. Bernie refused to meaningfully cooperative with the party who basically have a policy of allowing members across the widest political spectrum of any party, and then was shocked that they did completely legal and defensible things that might have affected his odds of winning the Primary, a Primary that was still ultimately decided by vote counts that he lost by a landslide. Nobody alienated Bernie (just look at Warren who writes half the bills he supports), he alienated himself.
Do people know why Bernie caucused with the Democrats? Because only about 9% of Americans identify as far left as progressive, and we only win something if we can compromise it with the only political party that works in good faith.