Our politics have also nationalized. Remember the old aphorism, "all politics is local"? For the latest decade or two, not only is that not true, but its erstwhile validity might be beyond conception for younger generations.

That doesn't get to the root of things, but the fact that our day-to-day cultural contexts have shifted dramatically is a much broader dynamic. I suppose a cheap answer would be TV, then the Internet, and most especially and recently social media. But that also only begs more questions--what underlying dynamics did those things effect.