A few factors - first, probably, diversity - Manga has a huge variety of titles and appeals fairly equally to girls and boys in Japan (and the US as far as I can tell). Second, you can get a lot more story for your money in Manga. Also, they really know their audience and provide reliable satisfaction of audience demand.
Finally, adaptation to Anime is a big factor. In the United States, if they make a Spider-Man cartoon or movie, it will sell more toys, clothes and merchandise than it will lead people to buy Marvel comics. Part of this is due to the fact that the movies and television shows are really unrelated to the comics. However, an Anime series usually looks like and follows the plot of the Manga - so if the series ends or you want to see parts of the story it leaves out, you can go to the Manga.
Personally, I also think Manga stories that are like superhero stories (you can say that Hunter x Hunter or Naruto are comparable to the X-Men, for example) develop better in comparison and often have a better sense of finality. When characters die, they usually die for good, and the protagonists develop and change without facing the same villains over and over (though sometimes it does feel that the structure of the plot is repetitive).