A lot of people are very religious and oppose abortion. And that is literally the only issue they care about. Trump stacked the Supreme Court and got abortion made illegal in many states. They love him for that.

Some people think he's stronger on the economy. They just think "look at all this inflation we've had under Biden! It wasn't like this when Trump was in control." Of course they can't think far enough to realize that perhaps a global pandemic caused this inflation and there was little that Biden could have done to stop it.

I think those are the big two reasons. But then there is the propaganda. People are made to believe that crime is running rampant despite the fact that violent crime is actually relatively low compared to in the past. Illegal immigrants are vilified as criminals who are bringing drugs and crime into the country while taking advantage of government benefits. The propaganda says that Democrats are the cause of these problems, and Trump is the solution. Tons of people fall for it. And Democrats are really bad at counter-propaganda.