If the Republicans are going to call the Democrats communists and socialist regardless of how moderate a campaign Democrats run, Democrats might as well lean further left on economic policy. Appealing to the right does nothing because the right can appeal to the right better than the center-left can
Which is precisely why they don't. They don't exist to threaten the establishment, they are part of it, and are there to provide the illusion of choice so that the public feel like we're helping while those in power do whatever they need to at our expense to keep that power. Playing by the rules and within the systems they have set out for us can and will only ever maintain the status quo.
Jangling keys of "maybe if I get the senate I'll raise minimum wage? Do you want... Legal weed sometime?" sure works well. Just look at that electoral map!