Isolation in the real world + insular communities stuck in an unhealthy feedback loop
It was easier to avoid being sucked into that kind of life in the pre-internet days, because you actively had to go out into the world and find those groups. It wasn't impossible, of course, just look at the propagation of right-wing militias in the 90s. They weren't using the internet, but people of like mind still gravitated towards each other. But without a doubt the internet, and specifically the ubiquitous nature of social media and the inability of tech companies to take action against blatantly poisonous groups, has exacerbated the issue.
