BBernbuam,
I don't know why not either.
I'm an American, and I must confess that I had never heard of the falls until I arranged a trip to Peru and Brazil earlier this month. Nobody I know had heard of them either. I am at a total loss to understand or explain this phenomenon.
Now I know that they're one of the most magnificent and spectacular natural wonders of the world. I was there for two days and visited both the Brazilian and Argentine sides. They were very crowded with people from all over the world.
I shall do my best to spread the word among my American friends.
Zwoopmugen,
I also visited Machu Picchu, and have no idea what you're babbling about.
Schrodinger's cat walks into a bar and doesn't.

What? Not famous? Are you north-american or something? They are very well known all over the world. I was there last week and met australians, germans, french, irish, etc. Not to mention latinamericans, who are more likely to know about the falls than seen the movie.
On the other hand, I'm kind of glad it's not full of ignorant retarded tourists like Machu Picchu (if you've been there, you know what I mean).