What Sanders doesn't seem to get is that Clinton has had years of connections to various organizations and has for years had a broader point of view on various issues, where as he's only be focused on millionaires, banks, etc. It takes time to gain support and that is how the process has always worked. And assuming that he does become the nominee, he has to work with these groups in order to win the election. Burning down bridges is stupid, but Sanders seems to want to do that.
What makes you think Sanders doesn't get that? It seems to me that this is akin to how anybody would spin his opponent getting an endorsement. What was he supposed to say? "Yeah, I really wanted those endorsements. That was a real bummer. Made me feel like a loser."