labrador's law: Only agitated dogs bother barking
The average post online gets something like 1000x more views than comments or replies. A post generally has to pass the viewer's threshold of emotional response to elicit the effort of a reply. Therefore, the responder either has a personal connection with the author or subject or feels strongly about it, and as such, the response is much more likely to be polarized, given the rarity of passionate moderation. This is a key contributing factor to online polarization: the majority does not feel as strongly about most issues as the people writing articles or posts about the issues. However, over time, saturation of the online media landscape with polarized posts and replies gives the illusion of higher polarization, which probably leads people to feel and believe that the issues themselves are polarizing and that most people ARE polarized on the issues.
Because we generally take sides in politics based on social acceptance rather than individual evaluation of the issues, the discussion becomes even less rational. If my social group is almost completely party-aligned, I will be much more likely to simply accept and promote the 11th-ranked plank of my party's platform than to depart from it, risking social isolation unless I deeply care about that particular issue.
This is why the most important issues are never the leading planks of a party's platform, but the most devisive issues ARE the leading ones, even if they are sometimes the least important, because that's how you gather support. Consider issues like abortion, racism and non-traditional sexuality. None of these issues affect more than 25% of people directly, yet they are hot-topic issues that drive emotional support of political parties.