It is because people with straight hair can have any kind of hairstyle imaginable. Perms do the trick nicely. You name the hairstyle, and you can have it if your hair is straight. Despite Hollywood trends, the fact remains that straight hair always looks good. People want to run their fingers through straight hair.
Curly hair is dangerous for fingers.
Tangle traps are everywhere. For a long time, I was terribly upset about by my curly hair. It still brings me down occasionally, but I've grown to accept it. And in case you haven't noticed, even Madonna is letting her hair misbehave in loose curls these days. But then, curly 'dos always look good when they are created by expensive celebrity stylists.