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
Oh, you mean Social Security? Medicare? Medicaid? Forgiveness from predatory student loan financial slavery? Overtime pay? Reasonably safe work conditions? The option to have a dead fetus removed from a woman before it literally rots in her and kills her? A basic education for every child, regardless of wealth? Healthcare for all Americans without death panel insurance companies who care more about shareholders than the policy owner?
You mean those things?
"But... but... muh property taxes and gas prices... damn blacks, queers, and loudmouth women ruin everything!"
You know who really doesn't give a shit about Americans, or anyone else but their "Tribe?' Nazis. Well, now you've got it. Hope you enjoy it when anyone you know who isn't white and straight is persecuted.
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!
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.