What is the cause of the split in western societies? I certainly sympathize with anyone who is puzzled about the current state of western democracies, especially given the rise in right-wing populism, as I am myself dumbfounded over this issue. However, I also believe it's something that we will never be able to truly understand in the same way we can acquire knowledge in fields like physics, because we cannot do any large-scale social experiments to see what will happen if we change just one variable. The best we can do is make some intelligent guesses.
From the history I've looked into, it appears that right-wing populism tends to take off when their is a feeling of economic insecurity present as well as insecurity regarding cultural identity, along with the further factor of non-responsive government institutions. I think, I am guessing, after the fall of the USSR,, many social scientists and political pundits thought that there was no alternative to democratic capitalism, so why bother addressing concerns that people living in these societies are having? They have no alternative but to accept the system. Only now we have come to realize how mistaken this thinking was ---- that there is the danger of fascism or some form of authoritarianism returning.
It's also my understanding that social media has a lot to do with the rise in authoritarianism. Social media uses algorithms that allow people to live in a bubble world, a self-reinforcing echo chamber, where they only see one view presented, and often it's a bullshit view of entirely made-up nonsense that people post on the web without really checking into whether the post is accurate, or meaningful in context.
It's simply one of those mysteries I doubt we will ever have a good handle on, except for maybe some broad facts that are present. We have history involved, social media, racism, educational institutions, political institutions, group-think, etc., etc., and how any of these factors contribute to the end result is hard to figure out.