Evidence strongly suggests that the showrunners felt obligated (or lacked the creativity to work around) a series of pre-set plot points determined by Martin from his book plans. At the same time, certain subplots and characters were removed from the overall narrative. The result was a number free-floating plot elements no longer connected to anything. Essentially certain Chekov's guns were put into play, but the characters who were intended to fire them, or the elements they were intended to be fired at, simply did not exist. Likewise there were guns that continued to remain on the stage even after they'd completely been emptied of ammunition (most relevantly, Cersei) because the persons that were supposed to clear them away were absent.
This actually reminds me, in a strange (and ironically relevant considering the showrunner's next project) way of the incomplete nature of the ending of KOTOR II, where you were so very obviously playing your way around a whole pile of cut content.