He is an institutionalist. Caucusing with Senate Dems gave him power, he can't then go about dismantling the very institution that got him to that point. He wanted to destroy the party without getting his hands dirty, but he should have just gone full bore and wrecked it. Of course the party was going to pull out all the stops. Hollow careerism replaced ideology, elected Dems don't believe in anything except themselves, so when Sanders comes along they see an existential threat to the party because they think 'I am the party!'.