From what I hear, American universities often prefer diversity in their student bodies, in academics, background, and so forth. This is a bit of a whimsical and broad generalisation, but Asian students tend towards the same maths/science subjects (or perhaps economics...) in their studies - it would seem to undermine this principle of diversity if a top university were to accept, say, a very large percentage of Asian students intending to major in the same areas. Hence the need to distinguish oneself with higher scores, or to otherwise choose a completely different mode of study (primarily in the humanities?).
That said, this is all inference/hearsay. I'll leave someone more qualified/knowledgeable (eg. appianway, people applying/who have applied to America) to clarify for me - in the mean time, there's a temporary guesswork response. :p