Because the leftist media and their accomplices have been spreading vicious lies about president Trump for 8 years. Some people are too stupid and lazy to do their own research and find the truth for themselves. You could not like Trump but there’s not a shred of evidence that he is Hitler or a racist. Trump was never called a racist until he ran as a republican.
You nailed it. If people who are ignorant to begin with are bombarded by lies 24/7, it’s going to have a major effect.