Because the really fast guys/girls from Jamaica, the US, Africa and Bahamas/Trinidad and Tobago are focusing on the flat distances, so only the less talented runners compete in the hurdles events and therefore you can make up for a lack of speed with great technique, like Shubenkov, Hejnova, Roleder, Pearson, Dutkiewicz and many more. If some of the elite sprinters would focus on the hurdles, than the white guys/girls would never again win medals there.
The same is true for the 3000 m SC, if some of the top kenyan/ethiopian talentes would focus on that event, then girls like Krause or Coburn would have no chance to win a medal.