cause not alot of people use it
Tiers are determined by how much people use them. Barbaracle is in PU cause not alot of people actully use it. Main cause of it being in PU is its typing( Rock/water). Its has a weakness to grass(x4), fighting, electric and ground. There all pretty common types.
In stats barbaracle is actully pretty good. Good HP, attack, defence and special defence too.
Tough Claws: Increases the power of moves that make physical contact by 33%
Sniper: Power of critical-hit moves is increased to 225% rather than 150%.
Hidden Ability (Available):
Pickpocket: When a Pokémon makes physical contact with the Pokémon, the item of the opponent is stolen.
Its ability are actully pretty decent but even if its pretty good other Pokemon are alot more powerfull then him. So since other Pokemon are more powerfull then him, he isnt used very much. So its stuck in PU.