The World Wide Web is a relatively new phenomenon. The first web pages were rather simple text documents. This event and the publicity that followed began to capture the attention of educators. 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.