Most publishers develop content in a waterfall-style model characterized by stages that are completed one after another with few loops in the process. This is the classic model used when developing physical based goods because of limitations with manufacturing and distribution that restrict the number of iterations you can have in the process.

For books, it can take two years from the time an author writes the first word until the completed book gets into stores (if it makes it to a store at all). There is a significant opportunity cost between the time the content is ready for consumption and the time it is available to be consumed. Internet publishing allows for a more incremental model. Release content as it becomes ready.