Likely because it's less risky for the publisher, as opposed to a big up-front payment for something that might not sell as well as expected. As an author, which approach would you prefer?
Residuals are fine if you can negotiate them. More power to you! But to complain about not getting paid for something without having to lift a finger to earn it is the height of conceit. Do The Work! Entertain us with new and interesting creations! The move towards rewarding work (such as the trends of musicians touring and performing instead of relying solely on recording) is a healthy one!