For they realized it's potential to teach. More recently web course development tools are beginning to be offered by commercial companies. Instructors often use print-based materials in instruction. Often without thinking even about it. A textbook has just a given.
Students usually expect a textbook.
This is because print-based media has surrounded all of us since childhood and in every class we have ever had we've had a book. This textbook then has become a necessary tool for teaching. The difference is that the web is relatively new phenomenon. Web-based course materials just do not have the tradition that textbooks have. Quite the contrary hypertext is much more useful.