Clinton lost because the DNC cheated to get her where she was in the first place... She was supposed to be the candidate that could never lose, and Bernie Or Bust isn't a real thing... Sure as heck was, I know Bernie supporters who voted Trump out of spite (mostly the independents) -- I wrote Bernie in personally, because I'd rather see the shit storm that is Trump -- piss people off so badly and ruin the entire reputation of the Republican party that progressive politics will come back in full force in 4 years. Than see clinton in office starting more wars we don't need and doing more regime change and taking money from special interests.
There's a real hatred for the elite class right now in America, some idiots thought Trump was that answer but he IS an elite. Bernie was the answer but the DNC thought they could quash him, now he's the most popular member of congress - and more progressives are running and winning than ever before.
2020 will be interesting to see if he runs again, if he does he'll probably be the next president. Even Nate Silver has said that Bernie is currently the front-runner for 2020 much as Clinton was in 2013.