I think it’s because people vote their identity. Here’s what I mean. People often think along this line “Someone I think is like me said to vote for a certain candidate (or buy a certain product, or believe something). So I voted that way because I identify with that person.” The messenger can be as important as the message.
Providing evidence isn’t the most effective campaign or marketing tactic. What seems to influence people more than evidence, are eager testimonials from other members of their tribe. People want to hear a story that resonates with them; it might not always be a true story. But if they can identify with the story-teller and the story that’s probably enough.
Before civilization took hold, going along and fitting in was bred into us generation after generation. You needed the tribe for protection; you wouldn’t last long on your own when there (really) were predators lurking nearby. If you were shunned by your tribe, you’d likely not last on your own.
But now there aren’t many life threatening events being a tribe member saves you from. But our built-in need to belong to a group is still with us and can lead us to vote against our best interest just to go along with someone we identify with as a tribe member.
Tribe members change their minds from time to time. When people who are respected in political, social or professional circles clearly and loudly proclaim that they’ve changed their minds, a ripple effect can start. First, peer pressure tries to repress these flip-flopping outliers. But if they’re charismatic and persistent, soon the majority flips. It’s not necessarily easy or fast, but it happens.
Now it’s hard to find people who still believe the earth is flat or that the sun is going away during an eclipse. Most people believed in those ideas until people they respected started insisting otherwise.
That’s why political parties change fundamental points in their platform now and then. It wasn’t that the majority reviewed the facts and made a shift. It’s because people they respected sold them on a new idea, a new opinion.