Because Bernie's a lukewarm progressive at best nowadays and he's incapable of actually influencing policy. The Democratic party is stuck in a rut caused by decades of neoliberal policy (which, for example, is why Clinton got so many resources during the Democrat primaries) and refuses to even consider a more radical alternative.

Voting isn't working to actually institute change in America. Either the country needs to push more power down to the states, or it needs complete electoral reform to remove the FPTP system that got America into this mess.