If you look closely since the return of the timeskip Oda has establish that to win a fight a swordsman have to have a stronger Haki than his oppenent.
Zoro who is overall stronger than Pica could not have beat him if his Haki was weaker.
And other pint of your haki is weaker than your opponent, your weapon get cut (i.e Zoro vs Hyozou, Zoro vs Apoo, Zoro vs Kamazo, Zoro vs King)
If a swordsman does not have a stronger Haki than his opppent he cannot hurt him and thus he cannot defeat him.
Zoro who is overall stronger than Pica could not have beat him if his Haki was weaker.
And other pint of your haki is weaker than your opponent, your weapon get cut (i.e Zoro vs Hyozou, Zoro vs Apoo, Zoro vs Kamazo, Zoro vs King)
If a swordsman does not have a stronger Haki than his opppent he cannot hurt him and thus he cannot defeat him.
Haki is the overwhelming determinant in fights because Oda is lazy and when he dies t want to think about fights, he yada yadas it to Haki. As demonstrated by the horrible Zoro vs king fight where Oda just gives Zoro CoC for the sake of ending the fight while forgetting to include the fact that King is invincible and Zoro never found a way to overcome it