My theory is that all this is the fault of the cookie law. Before that, the design philosophy was that you could not break the flow of a visitor by pop-ups etc., because they would go somewhere else before even looking at your content.
When all the big websites suddenly implemented increasingly annoying cooking consent dialogs, the flow was already broken everywhere. And so now the floodgates had opened for all kinds "subscribe to our newsletter", "get a welcome 10% rebate" etc., because users no longer has the expectation of an unbroken flow.
And, my god was that law stupid. What we needed was carefully balanced non-negotiable limits on what websites were allowed to do in terms of tracking users; what we got was every website implementing a site-dependent UI for functionality already present in every web browser ("turn off cookies"). The rules got different when GDPR arrived later, both for the better and for the worse. But the flow-breaking pop-ups we will probably never get rid of now that the public has learned to live with them.
End of rant.
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?