How did we arrive to this point where a huge chunk of the population is in one reality and another huge chunk of the population is in another reality? Segregation of information sources? Politicization of media outlets? Self-reinforcing social bubbles? A combination of all of them and more?
1) People prefer news coverage which confirms their previously-held beliefs (aka confirmation bias).
2) News outlets are financially incentivized to deliver news which confirms whatever beliefs we may have, since if we don't then viewers will change the channel. This causes news outlets to balkanize along various points on the political spectrum, ranging from extreme right-wing to extreme left-wing.
3) News outlets receive 100% of the gain of confirming our biases, but pay less than 100% of the total cost of doing so (aka "the tragedy of the commons").
4) Viewers end up consuming primarily (if not exclusively) media which confirms their biases. This becomes a self-reinforcing echo chamber.
5) Simultaneously, there has been a proliferation of media outlets. In the 1950s, there were 2-3 TV channels from which one could consume news. Today, there are 500 channels on DirecTV, and any number of internet news sites / Twitter accounts / etc. where you can get your fix.
6) In the old days, news outlets had to be moderate in their reporting, in order to appeal to the broadest possible audience. Conversely, the more sources of information there are, the louder any individual source must become, in order to stand out from the crowd.
7) With the internet, it's possible to create news outlets which cater to a minority of people who hold extreme viewpoints (aka capturing the long tail of the media market).
8) Over time, there are more and more outlets delivering more and more extreme reportage, and the news we consume becomes more extremist in nature.
9) Since our news diet plays a big part in shaping our worldview, over time we ourselves become more extreme.
I'd love to be wrong about this, since it doesn't bode well for the future of the human race. Someone please restore my faith in humanity by effectively rebutting the points above.