Prologing the show could kill it. AFFC and ADWD are no where near as well ranked as the first three books, and are drawn out, badly edited, and lack the number of events you need to spread out the show while at the same time keeping it interesting. Also, A Storm of Swords will most certainly have a lot of TV viewers cursing HBO, GRRM, and the entire series, and quitting. The books had exactly that happen, and the recent popularity is on the coattails of the show.

The fact is that the books are really, really drawn out in places. They get excessively worse as the series goes on, and they really don't need more than maybe 5 seasons at most for books 3, 4 and 5 (4 and 5 will be cut up and combined so they are in chronological order). The seasons need big events. That's TV. They need them. These events must also occur fairly close to the order of the books, or things won't make sense. So they have to find events to focus on or they lose viewers. Season 3, being only part (but the bulk) of A Storm of Swords, has probably the biggest event yet. Last night's episode was seasons two's big event. Eddard's death was season one. But after that... it gets REALLY ambiguous, especially since they have to cover the remainder of A Storm of Swords in season 4 (not much happens at all in that remainder), and some of the lead in to their book 4/5 chronological mashup. This doesn't leave them with many events to follow. And viewers will drop the show if they aren't entertained with major twists and events they expect to be there. So they have to move at a certain pace, and the most they are going to get out of books 3-5 is maybe 5 seasons. Maybe. At least 4, but there just isn't enough material to justify more than 5 without boring the viewers with trivial details, characters, and conversations they are already skipping over in favor of keeping the show streamlined and easy to follow. They've already cut and condences characters completely out of the show in order to streamline it for the audience, who would get quite bored if the show was less about events and plot twists and more about learning the massive genealogical backgrounds GRRM subjects his readers to without point or need.