It's not that they need to be popular, it's that voting them down leads them to be dropped off from view (and makes it less likely dissenting views will be shared). Reddit is the extreme case of this where anything outside the majority group consensus is heresy to be voted down/hidden/banned.

Better sites don't do this and have in-good-faith discussion despite disagreement.

Can you link some of these better discussions on better sites?

Dissenting views are regularly highly visible, often as replies to consensus views. Even better - well-argued counter-narrative/counter-conventional wisdom views regularly appear as top or highly ranked comments. That's because the people making those arguments do what sensible people do when making an unpopular argument - they put in the work to make their case more persuasive. They don't sit around complaining that the other kids don't listen to them, they care about their issue enough to try to work with human nature rather than hoping some magical technology will change human nature for them.