Because to them there are “good blacks” (the ones who went along with segregation back in the day, who might not have liked it, but were quiet about it) and “uppity blacks” (though they would have used a slightly different term).
Obama is uppity - they’re suspecting Carson, because he knows whose party he’s in and how they view anyone who supports Obama, is not. They’d vote for him. He’d still lose on a massive scale, though he would likely carry every state that McCain carried in 2008.