It's a difference in being familair with technology. It's a tactile experience with a book. When you have the book in hand, you know that inside those pages you'll be able to find something about your problem, or something about the system you're dealing with.
With the CD, you're more reliant on technology, however, you can store a lot more and transfer it back and forth to other systems when needed. Both have their place, but books are going out.