Dave Weigel's reporting on a gathering of black conservatives pretty much explains it:
This, though, is the limitation of the professional black conservative. The issue he or she talks about the most is not so much race as it is political correctness. And when he or she is asked to talk, it's in response to something the NAACP or a civil rights spokesman has said or done.
Let's say you're a conservative-ish black person who goes to church regularly, has socially conservative views, and is generally sympathetic to the "by-your-bootstraps-message" of the Republican Party. One glance at the GOP and it becomes clear that no matter how brilliant you are, your primary role is going to be inoculating the party from charges of racism. Meanwhile, if you join the Democratic Party, many of whose positions you might not actually share, you still have an opportunity to rise on your own merits, because black Democrats aren't really very unusual. Which choice do you make?
As long as the GOP is more worried about deflecting charges of racism against the party than it is about actual racism, this dynamic will continue to exist.
