I think this happens because people use darkrai in both doubles and singles. Dark Void is a good move for doubles. So they have to go to a move relearner and waste a Heart Scale to learn Hypnosis every time they play singles after doubles or vice versa. Also it is the signature move of darkrai which cannot be used against you by mimic or something (if you miss) untill the opponent is also a darkrai or has turned into one. And 50% accuracy is not that bad compared to 60%. Hench, they use Dark Void instead of Hypnosis.