Part of the reason pop music has settled on the 3-minute long track with lots of repetition and a simple hook is economics.
People are perhaps more likely to listen to a short catchy song on the radio than a meandering epic that takes 4 minutes to get past the intro. Probably whatever technology is cheaply available helps determine this as well. It also helps explain why the opening of the song just gets straight into the meat, because you need to get to the hook quickly to capture the audience.
I'm just expanding on what Freak said, of course.
By the way, has anyone read Godel Escher Bach? It has a lot to say on the matter of fugues and mathematical logic. Most of the book has faded in my memory by this point, I must say.