I will give kudos to AdAge for actually printing the truth about why this state of affairs exists, even if they did have to reach all the way to China to get someone willing to be quoted on it:
Lower pay scale
Advertising's lower pay scale vs. other industries is widely cited as a barrier to hiring and retaining minorities. "Advertising's centered in cities that have high costs of living and doesn't pay well in the early years. A higher number of kids who are not minorities can live off of mom and dad for the first couple of years," said Jason White, account director for Nike at Wieden & Kennedy, Shanghai. "The best and the brightest are heavily recruited out of college, and there's a percentage of minority prospects who figure advertising just isn't worth it."
Read the full article here.