Sometimes it's the professor writing the book so a new edition means more royalties for them. Other times it's like was mentioned in a neighboring comment, the

Sometimes it's the professor writing the book so a new edition means more royalties for them. Other times it's like was mentioned in a neighboring comment, the publisher offers some perk like an online tool to let students submit their assignments and grades them automatically. The publisher wouldn't have any interest in letting one book be used multiple times for that system so if you want to use it you need the new edition.