My explanation would be that the GOP was moving towards fascism for decades (dogwhistles etc.). Worsening of material conditions (effective QOL), propaganda or "talking points" on news and social media that became more and more detached from reality, fearmongering (wars, terrorism, islam, socialists, corruption, climate change) paved the way for him. He's just the symptom. Without the greed of the mainstream media to give his BS free airtime he couldn't have done it. But the time was right for someone like him because the old bullshit didn't cut it anymore. You see very similar all around the world (south america, india, europe, russia)
Normally the political system also heavily filters who gets to rise to power. He managed as an outsider because he was known.