I suspect it all goes back to Steve Bannon. He saw Trump as the type of guy he could use to propel his far right extremism after the far right lunatics appeared to go underground for almost 22 years after the Oklahoma bombing in 1995. Bannon saw a country sick of Democrat rule, and he knew precisely what the right wanted to hear. They were the perfect match. As a malignant narcissist, Trump didn't give a damn about the repercussions of pandering to the alt-right if it meant him becoming President. So long as Bannon was spearheading 2016 and schooling Trump on what to say, Trump was more than happy to ignore extremist views being normalised via the platform that was rolled out for them. Needless to say, Bannon was as happy as a pig in shit. Admittedly, not all Maga are extremists, but mimetics in Republican communities ultimately led to a whole load more than extremists jumping on board the Maga train. Trump (or Bannon) merely tailored his speak to keep convincing them that once upon a time they had something great that was stolen from them. Classic fascist trick. Throw a few wild conspiracy theories into the mix and you had a shit load of people really pissed off over having a time that never actually existed, stolen from them. It didn't take a whole lot of media discrediting to convince the already paranoid following that all mainstream media was a lie. Groups like OAN and Newsmax were only too happy to fill the gap in this market, regurgitating Trump's lies in mass supply. Wash, rinse, repeat and you have a seriously thick Koolaid mix.
One thing I am looking forward to in years to come, is somebody to write an absolutely thorough psychology book on what exactly happened to make Maga.