I think there are a bunch of factors at work here.
One factor is that young adults are under a tremendous amount of pressure to continue their education, even if the field that they're entering did not require higher education in the past. (In some cases those jobs do not require higher education in the present and going to college/university will set them back, but they are never told that.) This pressure leaves people willing to pay as much as they can afford to get that degree. Both universities and publishers are taking advantage of that.
Another factor is that textbooks are no longer just textbooks. Many universities seem to be hiring lower skilled and temporary instructors while increasing class size. This has created an opportunity for publishers, since they frequently provide everything from instructors manuals to online assessment tools with their textbooks. As far as I can tell, instructors manuals used to be answers to selected questions and assessment tools used to be a small question bank. Now they have everything from slides, to extensive question banks, to courseware with assignments and quizzes. Of course that courseware provides sophisticated ways to examine student performance. They'll also interface with devices like clickers that allow students to interact with the instructor in large lecture halls. Of course providing all of these tools costs money, which is passed on to the student.
I also have to question the time period examined. I seem to recall textbook prices being stable, and in some cases even declining, when I was in university (1996-2000). A prolonged period of stable prices can be marked by a period of high inflation afterwards since the increasing cost of materials and labour ultimately has to be accounted for.