Some are simply ignorant, and I don’t mean that in a pejorative sense at all: they simply get their news from what they think are legitimate sources and that’s all they know.
Others are selfish. They may like one or more of things Trump sells. That can be combined with ignorance, but may not be. Those people are simply selfish: I like it, and do not care if other Trump actions hurt people.
And then there are the utterly stupid. No need to explain anything there.