Left voters showed up to vote for him and then he basically abandoned the coalition he formed. He had a super majority and did like one, watered down Republican healthcare policy. He should've kept up the organizations and email lists he utilized to get into office to keep up the pressure on Congress in midterm elections, but that basically stopped as soon as he became President. He's not unusual in that regard, every President except for Trump, does this. But it's especially annoying for Obama because he had the closest to a broad coalition in recent history and it got him a super majority.
Bernie is basically the only Presidential candidate who has shown any awareness of how bottom up coalitions are used, not just to get one into office, but to continue to pressure with the power of the people. That's why socialists were excited for him. Because he would've been an organizer-in-chief. Every other politician rules the same standard way.
Well, Trump does a bit of the organizing thing, too, makes good use of the bully pulpit, and keeps up his movement between election cycles, but mostly to keep himself in power and enrich himself from them. Still, he's done a similar thing of gotten a lot of non-voters to show up, and pushed the party right. Democrats could do the same thing, and actually push the party left if they wanted to try, instead of showing open contempt for leftists, but it's admittedly harder because you'd have to fight the corporate owned media and pass legislation, or show you're actually trying hard to help normal people in public.