You are simply describing the effects of political and social polarization. I blame it primarily on a decades-long process of consolidation of wealth, influence and opportunity in the hands of an elite few, but no doubt there are other factors at play as well.

On the flipside I am very much opposed to any theory of the case that has it as being somehow uniquely American. It's not an American thing; it's a human thing that can happen in any country and has in fact happened in many countries throughout history. It does not require that we posit some kind of national hysteria that's unique to Americans when we can, with far fewer assumptions, simply point to polarization.