I am going to assume reasons for banning things have changed between the transition from BW OU and XY/ORAS. I wasn't in the competitive scene in BW, so I can't exactly tell you why it was banned, but I wouldn't make comparisons from then and now because a lot has changed, both in the game and in the council and their decision making.
Edit: Plus it sounds like you were just swept by some Cloyster holding King's Rock, in which I say, if a Cloyster 6-0s your team, there may be something wrong with it.
Edit: Plus it sounds like you were just swept by some Cloyster holding King's Rock, in which I say, if a Cloyster 6-0s your team, there may be something wrong with it.