Because Arnold Schwarzenegger says so.
This isn’t exactly a new prediction, but two recent news stories seem to confirm the downfall of the print textbook. First, the NY Times reports on a high school that ditched their algebra textbook in favor of a curriculum designed by the teachers. Then, the big news from California that Gov. Schwarzenegger has declared print textbooks to be old fashioned, and educators should start looking on the web for material to meet their needs. (My favorite headline about Gov. Schwarzenegger’s comments: Arnold Schwarzenegger says hasta la vista to textbooks.)
While California is hoping that the state will save money by not purchasing books for K-12 students, I hope that this trend extends into higher education. Wouldn’t it be great if students didn’t have to shell out $200 for their organic chemistry or introductory biology textbook? Instead, they could rely on free material on the web that their professor picked out?
Update: There is a new article in the Chronicle of Higher Education about one university’s experience with eBook textbooks.
the Undergraduate Science Librarian 
I am excited that someone is making the effort to reduce the amount of textbooks printed. This obviously has numerous advantage; less paper is being used, less fuel to deliver and ship these books,and students will learn in a more creative and interactive fashion. However I don’t think that this approach is all good. I think that the text book publishers need to adapt. Instead of printing books they should create websites that they can sell access too but still distribute the same material that would have been in a traditional text book. There are dangers to trust the material you find on the web. we have all been taught this. even the material posted by an instructor has not had the chance to be reviewed by editors for fact checking.