because he's saying exactly what a certain group of people want to hear.(this certain group of people being the incredibly conservative, racists, or(not and. i'm NOT calling all conservative people racist here) people who just don't understand that not everyone outside of America are terrorists. and yes, there are people who honestly believe that every other country is just out to destroy America. I met an old man who was scared to death because he found out a few Mexican families had moved into town(that's a pretty funny story by itself tbh).
The thing is though...he has no coherent strategy that I can see. Policy is all over the board, can't win at the % popularity he's got (and is doing nothing to correct that), and honestly has a lot of people wondering if he's a Democrat agent in disguise. It's almost like he decided to become the loudest, most obnoxious politician ever in some crazy Poe's Law experiment, got attention, and is now just too far in to back out.
I don't doubt that some actually agree with what he's saying. I'm sure a small % also like Trump and ideologically match his words if 3% of Americans can believe in Lizard people who control the government. I get the sense though that a lot of his support also stems from the "Screw both parties" individuals who hate the two-party system for whatever reason...be it hating political correctness that much, being more aligning with the Tea Party...whatever it may be.