Rural America is, depending on where you live, majority white, majority Latino, majority Native American, or some mix of the three. Generally a mix of at least Latino and white. Cities are segregated more by economic status than anything else (like cities anywhere). So if the working class of a city tends to be predominantly African-American or Latino, the working-class neighborhoods are overwhelmingly that ethnicity. But wealthy people of said race live with wealthy people, not with other people of said race. And poor white people live in the working-class neighborhoods (even if those neighborhoods are predominantly black, as was the case with Eminem growing up), not with wealthier white people.
Because he's multi-ethnic. If people had an aversion to America being a "melting pot", then subconsiously or consciously, they'd be less likely to vote for a multi-ethnic candidate.
My best friends in middle school were (1) an Israeli national whose family was in the US for a couple years; (2) a half-Iranian, half-English kid, (3) a seven-eighths Navajo, one-eighth Irish kid, and (2) a half-Japanese, half-Indian kid. I also hung out with with a couple of Chinese-American kids and about a half-dozen white people of various western-European ancestry. I myself was half Russian Jew, half-WASP mutt. This is not an exaggeration. And this was in Colorado, not the middle of San Francisco or something.