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?
The rioters/protesters are largely white as opposed to the more-diverse BLM protesters (more on that later). These people feel like the US Government, in general, and that includes moderate republicans, has failed them in the past several decades by sending their jobs overseas, or by allowing immigrants into the country who have taken their jobs. They feel like they're being left behind, which, in truth, they are. Blue collar jobs have been exported, en masse, over the years, and that's been a bi-partisan effort resulting in lifting countless people out of poverty outside the US, but putting incredible pressure on people inside of the US, particularly the middle class, who feel like they have few or no prospects.
And of course the irony here, when compared to the BLM protests, is that if a sizable segment of these rioters/protesters weren't racist they would understand that this is how Black American have felt _for a couple of hundred years_. Important note: this is not to say that these rioters/protesters are even close to being as poorly treated as the black community. Outsourcing their jobs and giving their former wages to owners/capital in the form of profits is SO FAR from the historical treatment of black Americans that someone will read this and consider it laughable to even compare the two. But they are getting a taste of what it's like when the government stops working for them, the formerly very privileged (not relative to the 1% but certainly to black americans) white middle class.
All of the polarization stems from that. "You've taken my opportunity and everyone else seems to be getting theirs and I no longer have mine. Fuck you. I'm going to break shit until you listen up."