A part of that comes from this being the modern world, 180 years ago immigration was more challenging and permanent (not a plane flight, it was one way, bridges were burned), and America was truly wild and dangerous, yet it was already the freest, greatest, most noble place for the brave men and women who could qualify, to be.
Today it is about money, the globe is full of centers of immigration for economic gain, now it is largely about who has the easiest entry rules, and the most free goodies, America and Britain, France, Sweden and other Western nations win in that category of combinations.