well, the main reason is that most of the good stuff was found long ago; the simple harmonies and melodies of Mozart or Beethoven were great just the way they were. Anything of the same nature people would come up with nowadays will just sound like a cheap imitation. Therefore, they have to keep coming up with new, strange, not-so-easily-likeable innovations in music, that generally lack the immortality of the past's great work.
I think it also has to do with living conditions; 19th century Europe was rich in historical battles and revolutions. People were dying everywhere, and making love in the haystacks like there was no tomorrow etc.. (EDIT: ok, maybe a
bit of an exaggeration, prometheus

)
but nowadays, we only have boring problems to worry about, like terrorists or global warming. Who the ^%$ is going to compose a good piece of music about global warming?? Again, the fact of the matter is that the most interesting stories and historical events have passed by long ago.
For inspiration, we simply don't have the prolific masterpieces of Goethe or Victor Hugo nowadays; instead, we have petitions for gay rights and feminist movements!
phhhaaaaa!!!From a musical point of view, the world is getting bored and constipated. I'd like to give it a BIG enema; let it exhaust it's non-renewable resources so we can go back to the horse-and-buggy days. It would suck to live in this time, but I think the art and music will blossum once again.