The question seems to be: why has a substantial percent of the US population been captured by misinformation, disinformation, propaganda, conspiracy theories, and magical thinking? IMO the answer includes the below three elements.
- People susceptible to propaganda: Segments of our population were educated before social media and the internet and do not have a grounded understanding of how to digest and process information put in front of them. There is also the low educated who weren't able to pick up the needed critical thinking skills and media skills. They do not understand the methods of weaponized propaganda. They do not understand they are in an information bubble.
- Weaponized propaganda: The GOP and right wing powers are well funded. They have the ability to craft narratives and push those narratives with a high degree of effect. Their narratives are pushed by right wing leaders and echoed throughout the information landscape, including talk radio, cable news, twitter, facebook, youtube, podcasts, reddit, and religious groups. These narratives use fear-based identity politics (immigration, race, socialism) and wedge issues (abortion, guns) to tap into people's genetic predisposition towards tribalism. Algorithms, social media, and personalized advertising amplify this effect. Think Cambridge Analytica. Troll farms pushing these narratives through social media have been a thing for years. Foreign actors also play a role in funding and pushing these narratives. The "vast right-wing conspiracy" directed at the Clintons is an early elementary example of this.
- Information bubbles: People allow themselves to only be exposed to information that supports their preexisting ideas. People experience positive brain chemistry when presented things supporting their beliefs. It is hard and uncomfortable to be challenged; thus, it is easier to avoid counter information than it is to consume and critically consider. Information sources link each other giving the false impression that ideas are widely supported or accurate. Certain search algorithms exist (youtube) that push people deeper into rabbit holes.
A succinct answer is probably something like the right uses weaponized propaganda to control people susceptible to propaganda and place them within media bubbles that then result in large populations being within alternative realities.