I think the most likely reason that the change was made in Sun/Moon was to stop two really cheesy strategies in competitive Pokémon play, namely Dark Void Smeargle and, to a lesser extent, Voidcats.
Dark Void Smeargle was a huge pain to play against, particularly in the ORAS VGC season when it was allowed in both online and official competitive VGC matches. An 80% accuracy move that can put both opponents to sleep is borderline overpowered, especially in Doubles where battles move really quickly and a single turn lost to sleep can have a dramatic income on the outcome of the match. In theory, Dark Void Smeargle has a lot of counters. Fake Out, Prankster Taunt, etc. are effective ways at beating Smeargle in theory. But, in Doubles, Smeargle has backup. Usually a really fast Fake Out or Taunt user like Mega Lopunny or Jolly Kangaskhan, or even Quick Guard to block Thundurus or Sableye's Taunt. As a result, when teambuilding you usually needed multiple counters to Dark Void Smeargle, and it had quite a strangling effect on BO1 competitions or 1v1s. So, it was nerfed coming into Gen VII (where Smeargle is part of the PokéDex), along with other oppressive aspects of the metagame like Prankster and Parental Bond. The accuracy was nerfed to 50% and it can only be used by Darkrai (personally, I think the accuracy drop was a bit too much).
Voidcats is a niche strategy that uses Liepard's Prankster Assist to select Dark Void, meaning it got a +1 priority boost, which is crazy in Doubles. I won't explain the whole strategy, but basically it abuses Liepard's priority Assist to select Dark Void (accomplished by having every other Pokémon's moveset consist only of moves that Assist cannot select) to keep the opponent in a state of constant sleep, then have other Pokémon use the few offensive moves like Fly and Focus Punch to claim victory. This strategy has plenty of weaknesses, but can catch an unprepared player by surprise. It's more of a nuisance than an actual winning strategy — nobody serious would take a Voidcats team to an event like Regionals without some serious guts — but it's aggravating to fight against.
I highly recommend reading this report on a Voidcats team that won the National Championship Multi Cup. Using Voidcats in multi battles is a little easier, because only three of the Pokémon's movesets are limited by Assist.
TL;DR the mechanics of Dark Void were changed to stop cheesy and annoying Smeargle strategies.
