From AFFC and ADWD on the plot just gets to big. Characters disappear or are rumored dead for long periods of times. Several new main and side characters start appearing rapidly. More locations and cities are added. On there limited budget its going to be hard to pull off.
I still agree with this. Martin has to crank out some books fairly quickly or the series is going to catch up to the books too fast. There is no way all of affc or dwd is going into television. There is already a consolodation of characters going on in the first two seasons (no Bolton?). Apparently after asos the action cools down dramatically and we are treated to an entirely brand new story with entirely brand new characters. I don't see television readers being as forgiving of such excess as book-readers. The whole iron-islands/dorn thing seems to me to be more like a spin-off series than anything nescessary to the main story its self. Like, ya its important to the plot of the Cunningham family that we know all about Joanies marriage and her baby, but really we don't screw up Happy Days by making an entire three seasons JUST ABOUT THAT, instead we have a spin-off series so the main story can continue on, flow uninterupted.