Think of this from his perspective: the Democrats put their faith in the idea that money wins elections, and if you can out-raise your opponent the votes will follow. Twice they conspired against Bernie in the primaries because of his platform: tackling wealth inequality, progressive tax reform, and overturning Citizens United v. FEC. They chose corporate interests over the working class because they valued money more than votes.
If you get voted into power, lobbyists give you even more money.