College professors: Why do you generally have your students buy very expensive textbooks?

As is the case with most students, many of my professors require expensive textbooks for the class they teach, but I have a few who require the kinds of books that you can pick up at a regular bookstore. These books are much less expensive and as effective as the expensive textbooks. However, many professors choose not to use them. To those professors on fluther, can you tell me why you choose expensive textbooks rather than these less expensive alternatives?

Note: I know that some fields are so specialized that it would be very hard or impossible to find a book in a regualr book store that covers all the material but this is not necessarily the case with a lot of fields.