Same reason why SUVs are so popular I guess, people just like it.
Plus anime is by far not only for children, in fact there is a lot of stuff I wouldn't show to children. Bad thing is, the shows broadcasted outside Japan are mostly childrens' shows and/or horribly dubbed, that's why most people resort to watching the fan-subbed versions. Another point is, with animated shows you have a lot more freedom for writing the story, simply because more is possible with drawn character than with real actors, and it's cheaper than hollywood stars too.
Bad thing is: The most popular animes are usually the worst ones, see Naruto. I personally find Naruto is a good example how to do it wrong: Too flat storyline, too many childish jokes, too long, too much filler episodes and gradually worse image quality. I wonder just as much as you why it could become so popular, just like with the SUVs - no good points, but still popular...
I hope you can understand the download-ratios a bit more now