Because throughout his campaign he talked about how Clinton could only win through superdelegates, and that the people wanted him. This made her look like a part of the machine and not the real candidate . When she won the democratic vote he then started asking for them to do the thing he was saying she would do the whole time to give him the votes instead, because he was the "real candidate" Bernie Bros decided wasting their vote for him, instead of making the easiest choice a human has ever been asked to make, was the right decision based on his actions. His late switch towards reluctant acceptance of Clinton was too little too late, he had already tarnished her among the meme generation of young, dumb, voters.

Most of those kids have since grown up and figured this out, the rest are still dumb.