The discussion thread can still be found in the OU forum, if you wanna read a lot of arguments ^.^
Basically, you HAVE to pack a dedicated counter to it (most of which are garbage, and those that aren't, such as Roar Mega Gyarados, are usually suboptimal), if you don't, you simply lose at team preview. If you're badly prepared for playing against, idk, stall, a good player can still work his way around it, but that's not possible with BP.
Haze is garbage outside of stopping BP and there are no OU viable mons who can afford to run it, Taunt is bounced back by Espeon, idek how Infiltrator is relevant, hard attacking without stopping is not as easy as it sounds when they have their defense boosts up, etc. Even unaware users don't work, they ignore the sp.atk boosts but they still have to eat a 500 BP Stored Power. And if you don't pack any of the few counters, you lose at team preview and you can't work your way around it, which takes skill out of the game. That's basically the arguments for banning it ^.^
(and you can't simply say 'run haze lol', because then we might just as well unban Kyogre and say 'run Gastrodon lol'. And nobody is forcing you to play by Smogon rules, if you don't like the BP clause then agree with your opponent not to adhere to it ^.^)