Actually, there IS a strong demand for ebooks with rich media-but its not reflected in the Kindle Top 100. Amazon's patrons typically want narrative text genre fiction-the cheaper the better.
Where is there a demand for rich media ebooks? In the iOS App Store.
If you browse the Books and Education categories, you will see thousands of titles of children's book apps. As I posted in an earlier thread, the print children's book is being driven out by children's content apps, with the App Store leading the way. See , for example,
NOSY CROW.
The children raised on these apps are going to go to K-12 schools where they are going to learn from digital textbooks produced by (surprise!) Apple, which will also be rich media. No prizes for guessing what kind of textbooks they will be using when they get to college.
I think there will always be a demand for plain vanilla narrative text. But its also a low price, low reward game. I think Apple (and maybe Microsoft) want to go where the margins are higher.