Best explanation that I'm aware of:
The 1950s were an intellectual monoculture born of 2 world wars, the great depression, and a huge consolidation in media outlets. First newspapers, and even the new Television-jobber were all, by 1950, controlled nationwide by very few players. Not only were we "all in this together", but there were also only 5 all-identical but for the capitalization, inbred opinions available in public. Everyone basically agreed. FDR-ish fascism was the way it was.
Increased Segregation. Whereas the prior 200 years were all about people from different backgrounds, and of different opinions mixing together, and finding underlying humanity...Sometime after forced bussing, more well off folks started isolating themselves into entirely-like-minded communities. Some evidence suggests that the probability of a member of your zip code agreeing with you politically has increased from 50% in 1950 to something like 60% today. And neighborhoods are likely more so. My thought experiment is: what level of tolerance is generated if your next door neighbor with whom you barbecue on the weekends, and with whom your children play happily has a different politics than you? I assert that you learn to tolerate/respect those of different opinions than your ownI have a third explanation as well...that i don't hear often enough:
Strong fences make good neighbors. If I send my kids to a private school, and you send your kids to a private school...then we're both happy, and mostly happy to let one another educate our kids as we please. On the other hand, if we both educate our kids at a tax-supported public school, then our very different proclivities will lead to increased vitriol. This extends further. If the federal government participates in education decisions, then you and I, even though we live in different states, have reason to fight with one another. Contrarily, if I live in Texas and my sister lives in Portland, and Borepatch lives in Atlanta, and the states control their education entirely...then Borepatch's neighbors will have the public schools teach their kids that God made America, my public schools will teach that gun control means tight groups, and my sister's neighbors will have their schoolchildren offer incense and crystals to Gaia. And we'll all be happy. The more of life that governments control, and the larger the unit of governance that controls it, the less civility will prevail.