Previous research on textbooks and teachers’ use of textbooks shows, among other things that:
Math problems and topics are being taught in school and by professors through textbooks. Textbooks provide written direction and step by step shows the student how to solve the problem.
“Mathematical topics not included in textbooks are most likely not presented by the teachers (Freeman & Porter, 1989; Reys et al., 2003);”
According to the article Teachers and Professors use textbooks as a primary source of information. If you want to find more about why and how professors prefer textbooks for class discussion read this article.
Educations digital trend 