What do you mean by conqueror haki? Unlike other types of haki like armament or observation haki, conqueror haki is only found in a few people, who have strong will, determination, and commitment. The act of never giving up is seen in the conqueror haki users., No matter how hard their path is, they have never given up. These people have conqueror haki depending upon the user’s sheer will.
One-piece chapter 1047
here is the real deal
here, Kaido explains what haki is . Haki is the only thing that counters devil fruit and kaido mentions that abilities like devil fruit cannot alone conquer the world. and Pirate king Gol D Roger was not a devil fruit user. All he had was a strong conqueror haki in order words he had courage, and firm determination that no one could compete with him. so only haki can transcend all.
So Kaido, Big Mom , whitebeard, and others all have conqueror haki but why couldn’t they surpass Roger. The main reason why they couldn’t do is they gave up when things got hard. They all looked up to him. but they gave up on the way that caused them to steer their path. When you look at kaido’s past, you see he wanted to be a joy boy but he ended up slaving and killing innocent people of wano ,he approached the wrong way to be a joy boy so he gave up.if you see big mom, she too wanted to create a land where everyone is happy , singing and dancing but she ended up creating a kingdom where she took souls as a tax from them and all the citizens had to live in fear. So big mom too gave up eventually. whereas if you see luffy, you sense a rebellion, everywhere he goes he brings joy to the people living there, no matter how hard the path and how tough the enemy is , he will never back down, he never backs away from his dream. Thus, Luffy has the strongest conqueror haki in One Piece and he is the one to surpass the pirate king Gol D. Roger.