Why is Wizards doing this to us when they could be releasing the eleventy-millionth Forgotten Realms sourcebook?
Every time Wizards of the Coast releases a product within one of its existing worlds, the company is hoping for audience depth: their available market is mostly limited to existing customers. Creating a new campaign setting, especially when it’s promoted with books and computer games, gives the company a chance for audience breadth, creating new customers. Besides, WotC will still support its classic settings. If anything, more customers and more money mean more funding for the campaign settings you know and love.