I think you have pinpointed the core issue.

Right-wing republican policies and ideas lends themselves to simple (but often wrong) models of explanation; "it is the fault of the immigrants; the poor; abortion is always immoral", etc. You get candidates that radiate confident leadership spewing simple talking points they believe in.

Left-wing, especially progressive, ideas are often rooted in insight into the incomplete understanding we have of the underlying complexities. People who navigate these ideas won't be as confident: "the cause is a bit of this and a bit of that; we don't really know, but research points at" etc. To confidently sell policies based on these ideas to voters requires a level of cognitive dissonance, and also opens for criticism on being indecisive.

How can we package left-wing ideas in a way that attracts voters who are swayed by simple ideas presented with absolut confidence?