Not because Doran died, it's a logical move given in the book Aegon looks like he'll be a dud, and his son already struck out. So cutting Doran out was expected, because he's probably going to get toasted in the books. But god damn, they just shit on his character completely. Why get such a great actor and even invest any time into him? That whole plot felt like it could have been much more athletic without him...I suppose they need the Sand Snakes since they are all over in the book, but the whole Doran thing was a waste. In fact, Dorne itself was a damn waste, it should have just been the Sand Snake story line in KL.
The most epic speech...
"I am not blind, nor deaf. I know that you all believe me weak, frightened, feeble. Your father knew me better. Oberyn was ever the viper. Deadly, dangerous, unpredictable. No man dared tread on him. I was the grass. Pleasant, complaisant, sweet-smelling, swaying with every breeze. Who fears to walk upon the grass? But it is the grass that hides the viper from his enemies and shelters him until he strikes. Your father and I worked more closely than you know . but now he is gone. The question is, can I trust his daughters to serve me in his place?"
Gone. Would it have really been that hard,ifyou were going to have him, to have him die within the process of some machination rather than simply looking like the weak man the idiot Sand Snakes thought he was? They could have easily still made him a deeper character while making the Sand Snakes win. Or, as said, simply not have him at all, the whole Dorne plot should have been contained to the Sand Snakes, off Mycella back in KL through a Sand Snake plot there. Why go to Dorne?