Because the work to create one of these, particularly if it's work that involves updating the Adventure Path to a new edition, is not insignificant and Paizo isn't a large enough company that we can take people off our ongoing lines to update older content like this all the time. That, plus customer interest, our own interest, printer and warehouse capacity, stock levels of existing product, and more help to ensure that the rate at which we are currently producing hardcover compilations is pretty much what we can do.