Because they appeal to racism and bigotry. Do you know anything about the political environment in America?
Our former president LBJ put it quite well "I'll tell you what's at the bottom of it," he said. "If you can convince the lowest white man he's better than the best colored man, he won't notice you're picking his pocket. Hell, give him somebody to look down on, and he'll empty his pockets for you."
That's how the Republicans win with lower economic groups. Racism against minorities, bigotry against homosexuals, sexism against women. Republican economic policies only hurt the middle of lower classes dramatically. But LBJ put a quite well how to win their votes.