There's no need to explain why, it's frustrating to play a PvP, be about to win, you still got a counter vs Cloyster, and then, from nowhere, when cloyster just sets up Shell smash, tada, you get flinched!
It's literally one of the most unhealthy PvP items. For those who say that "King's rock is ok 'cause paraflinch exist eksdee", paraflinch can be stopped by lots of things like natural cure users, electric types, or heal bell cleric users, while king's rock cloyster is just a spammy
It's very rare to see high usage as an indicator of the strength of a particularly problematic Pokemon. Even some of the most oppressive threats in the game's history have had pretty low usage, the thing about usage is it mostly indicates the splashability of a Pokemon IE in how many teams this Pokemon can function properly. And this segways perfectly into Lucario, Vacuum Wave Lucario is by no means a weak Pokemon, with proper support it's a fairly strong pick, but it's sadly not a very splashab