>There also wasn't this huge epistemological divide running right through the middle of the country; people were generally working from a common understanding of reality. Not so, today.

Everyone was watching the same network news and reading the same newspaper. Once niche news sources started to become widely available, everyone went to the news source that gave them the news how they wanted to hear it. Over time these news sources grew more and more polarized as they continued to cater to their specific niche audience (some sources are certainly more brazen and intentional with this approach, but it happens naturally if you aren't actively working to stop it). There is only so much polarization that can happen when we all agree on the same basic set of facts. But now we are all stuck in our own bubbles, only listen to news that reinforces our world view, and we continue to spiral further and further into polarization.