Because US people are incredibly racially polarized and almost any topic is prime for it to be used as a leverage point to imbue their racial polarization on
It looks more to me like a lot of rich people doing their own thing seeking profits, and the result for the rest of us just ends up crappy as a by product. Traditional media makes a ton of money from outrage, so they forment and spin outrage, not to divide Americans (except as a by product), but because it makes them so much money.
They are two sides of the same coin. There is no single entity creating this atmosphere, just a bunch of outlets amplifying the most radical voices, as normal news and balanced debate is boring and not that profitable.