To be fair the reason he lost in the primaries has less to do with his supposed left leanings. The Media said it was over before Iowa even voted, by counting in Super Delegate votes. The media, gave five Republican candidates—Trump, Jeb Bush, Ted Cruz, Marco Rubio, and Ben Carson—more coverage than Sanders during 2015 and that Clinton herself received three times as much press.
The DNC completely changed their format of debates, to expose Clinton less, and to minimize Bernie's Exposure. In the Russian Hack, DNC emails showed that Clinton received questions to debates prior to the debate, the hierarchy of the DNC was in the bag and pulling whatever strings it could to help her along.
It is what it is now. I vote for a lot of democrats because they more so say the believe in the things I believe in, but have voted for republicans as well (many times).