I'll let David Frum explain,
When you ask white Americans to estimate the black population of the United States, the answer averages out at nearly 30%. Ask them to estimate the Hispanic population, and the answer averages out at 22%. So when a politician or a broadcaster talks about 47% in "dependency," the image that swims into many white voters' minds is not their mother in Florida, her Social Security untaxed, receiving Medicare benefits vastly greater than her lifetime tax contributions; it is not their uncle, laid off after 30 years and now too old to start over. No, the image that comes into mind is minorities on welfare.