With a runtime of over two decades, One Piece is one of the most well-known media franchises in the world. Therefore, it wouldn't be surprising if fans sometimes feel out of depth when it comes to remembering all the series' history so far. With so many plot lines to keep track of, the smallest details can easily slip out of sight. While One Piece's devil fruits are unique to each individual who consumes them, haki is a different story altogether. As long as people train themselves well enough, they can master the art of haki and bring down powerful enemies.
Strong pirates all over the world and even the marines' highest-ranking members are extremely skilled at using haki. However, there is one rare type of haki that only a few people in the world are born with: Conqueror's Haki. Unlike the other types, Conqueror's Haki can't be learned through training and is only possessed by those who are born with the spirit of a king. Thus far, only a few pirates have been shown to use this haki. Therefore, the question remains -- why can't the marines use Conqueror's Haki despite being so powerful? Eiichiro Oda certainly has a good explanation for this since he doesn't leave any plot holes in One Piece.
The Marines' Questionable Sense of Justice in One Piece
There's no doubt that the World Government and the Marines are the largest antagonistic organizations in One Piece's world. They commit several heinous crimes while being fully aware of how wrong it is, yet they hurt innocent people in the name of justice. Doflamingo once questioned their very sense of justice during the Marineford War -- that only those in power get to decide what justice is.
Taking Akainu for example, one can see how he views himself as a medium of destruction targeted toward the evil that is pirates. He is aware of how strong he is and believes what he's doing is right, without caring about who gets hurt in the process. The rest of the marines are like that as well; they follow orders without caring about the consequences and collateral damage. The tragedies of Ohara and Flevance are perfect examples. They would strike down even defenseless children for the sake of their questionable sense of justice.
Why Are the Marines Not Worthy of Wielding Conqueror's Haki?
Conqueror's Haki allows the user to exert their own willpower over others. In the case of marines, even the strongest ones, using this type of Haki is out of the question since they are used to following orders. For example, Kizaru prefers being told what to do, even by people who rank below him such as Sentomaru. He is completely fine with being a tool for the World Government to use at its disposal.
Even powerful marines such as Garp joined the marines and forfeited their chance to truly rule of their own accord. That is why the marines cannot use Conqueror's Haki, which signifies the will of the king. They are bound by rules and regulations, unable to live freely. As Luffy explains, being a king means being free, which is not the case in such a powerful organization.
Is there an Exception to the Conqueror's Haki Rule?
Up until recently, a marine using Conqueror’s Haki was completely unheard of in One Piece. Not long ago, however, Oda revealed some new characters -- including Sengoku, the former Fleet Admiral of the entire Marine force. Thus far, he is rumored to be the only known marine who has the Conqueror's Haki. It could be due to his motto of reigning justice, which shows how he wishes the justice of the marines to reign over the world like sovereignty.