Credit to I'm Rick Astley for going through this with me and helping me write this up
Mega Sableye is a huge problem in the OU metagame right now and for a bunch of different reasons. It heavily centralises the hazard game, shutting out so many hazard setters and forcing balance teams to run SR Clefable if they even want to get rocks up against it. The support it provides to teams is incredible too and when you couple that with its ability to shut out almost all hazards and support from teammates, it's insanely restricting in terms of handling it and has such a large strain on teambuilding.
Essentially, Sableye makes stallbreaking a much more matchup-based process than it ever was. What happened before was a player could pressure a stall team down by setting up hazards and double switches. If you predicted a switch in to something, you could double out into the appropriate threat, and get 12.5% on whatever just came in, which could be crucial for bringing it in range of being 2hko'd on the switch in. Now, predicting a switch becomes useless if there is no reward for doing so, which is what a hazardless game vs a stall team does. Without the opportunity for hazards, you are simply left with the 6 pokemon in your party, and what is worse, they all work as individuals, rather than as a team. What I mean by this is that one pokemon which can effectively remove checks and counters for another does not work well if the opposing player can just keep switching out into their counter for each with no repercussions.
ABR's Mega Sableye team, which everyone reading this should be all too familiar with now, is being spammed both on the ladder and now even in tours. Players are using it exactly for the reason that it requires little effort to use, is heavily matchup based, and will win a very good portion of matchups due to how effective it is. So, how is the stall team losing a match at all? It is done by specific breakers, rather than smart play or team synergy, because those don't mean much without the capability to wear down opposing pokemon so that another can do work. Once you have those specific breakers, you can then use those effectively in order to win. However, a bad matchup vs this sort of Mega Sableye stall team is almost the same as an autoloss, disregarding the chance for hax to happen. This makes Mega Sabeleye very uncompetitive, under this definition:
II.) Uncompetitive - elements that reduce the effect of player choice / interaction on the end result to an extreme degree, such that "more skillful play" is almost always rendered irrelevant
This is the crux of the issue with Mega Sableye, by controlling the hazard game in the way that cores involving it do, it isolates your pokemon, then enables the rest of the stall team to completely negate the effectiveness of most of them. This lets stall players make obvious plays without the opportunity to be punished, because there is no penalty for having to switch back into the appropriate counter if the opposing player doubles on you.
What you have here is a pokemon that forces whatever specific breakers are effective against the stall team in question to be used in order to not autolose, and takes away the penalty of predictability in damage on switch in. It is also worth noting that due to the large variety of pokemon that Gothitelle can cripple, the pool of pokemon that can be used to beat a Sabeleye stall team becomes smaller still. However, merely banning Gothitelle does not prevent the conceptual problem, whereby you either pack the correct breaker for the correct stall team, or autolose, and no amount of smart play will be able to prevent this. It is for these reasons that we strongly believe that Sableye is uncompetitive in the current metagame and deserves a suspect, then subsequently a ban.