1. Time Constraints
One of the primary reasons why the show has begun to make significant deviations from George R.R. Martin's work is that its creators and writers have just ten hours of programming to fill each season. As a result of this, they have to cull significant amounts of content from Martin's books - A Storm Of Swords, for instance, clocks in at 1236 pages, with its audiobook version taking 47 hours to finish. This means that even over two seasons (20 hours), there are still many hours of content that have to be cut. An assortment of great and important content has already been lost in the transition to screen, most notably details of Robert's Rebellion and its key players, such as Rhaegar Targaryen and Lyanna Stark, as well as the series' assorted prophecies and visions, which play a huge part in the book's overarching plot. In addition to this several important supporting characters have disappeared, either completely or into the background, whilst others (like the mighty SER POUNCE!) have not yet been introduced despite appearing in the books by this point. Everything will have a knock-on effect on season four and beyond, where the question must be asked as to why HBO don't see fit to expand their season orders to thirteen or fifteen episodes.