Because Yahoo! is discontinuing their Shopping API.
After careful consideration, we have decided to enter into a strategic partnership with PriceGrabber to power the Product Submit functionality of Yahoo! Shopping as of March 11, 2010. As a result of these changes, Yahoo! will no longer provide the Shopping Web Services API, including Shopping Results (the “Yahoo! Shopping Syndication Services”) to you as of March 11, 2010.
If you wish to continue to display syndicated shopping results for products listed on Yahoo! Shopping, you must apply to PriceGrabber for shopping syndication services. Although they do not offer a free web services API, you can find out more about how to apply to their program here.