Honestly?
There are several reasons, some legit, some nonsensical.
One of the reasons that they're so much more expensive now than they were a few years ago is because of all the "stuff" that comes bundled with them. Study guides, CDs, DVDs, websites - all of these things cost the publisher a lot of money to create and those publishers aren't the least bit shy about passing those costs back to you.
Another reason is because the books aren't purchased in particularly large quantities, though it may seem like it. At some colleges, the 6 different professors teaching an Intro Psychology class might assign 6 different textbooks. It would save the students a ton of money if every school in the state university, for example, used the same Intro text. That way, the state would order thousands of the same book each year and the costs would go down.
Another reason is because the publishers come up with new editions almost every year - they'd tell you that they want to keep them updated with the latest information - but students will tell you that it's only because they want to make last year's book unsellable, to force every student to buy a new book.
Yet another reason - the textbook publishers don't divulge the pricing of their books to the faculty when they send them free evaluation copies. A professor looking over a new text has no idea what the students will be charged for that book if he/she adopts in for their next course.
Here's some good news - the US Congress feels your pain, and the House of Representatives has passed a bill that addresses the high cost of college textbooks. Now it's in the hands of the Senate. If you want to see it passed, you and your friends should contact your state Senator about the College Opportunity and Affordability Act.
Some of the things the Act does is
require publishers to disclose textbook pricing and updating information to professors
requires publishers to offer textbooks and their supplementary materials separately (unbundling)
requires colleges to provide the list of required and recommended course materials at the time of registration
Connecticut, Washington, Oklahoma, Minnesota and Oregon have already passed their own state versions of this legislation, while California, Massachusetts, Arizona and Colorado are considering similar laws. Again, contact your state legislator, and urge them to take action against the prohibitive pricing of college textbooks.
Finally - here's a website for an organization that is working to battle the textbook pricing mess. You'll find lots more information there. I hope this helps!