Because of this major change.
I just noticed Dark Void's accuracy was reduced to 50 percent in Generation VII (Pokemon Sun and Pokemon Moon). That move had an accuracy of 80 percent in previous generations.
To top that off in Generation VII, that move can only be successfully used by Darkrai or a Ditto that has transformed into Darkrai. That move will fail if any other Pokemon learned that move (Smeargle), or ties to use that move via Assist and Metronome.
That change will no doubt demote Darkrai from the Uber tier, and place it on the OverUsed (OU) tier.
I can now use Darkrai without feeling guilty, because its sleep inducing move has the worst accuracy in the metagame; for example, Hypnosis has a 60 percent accuracy, and Sleep Powder has a 75 percent accuracy.