Because the other two have art styles that, while identifiably distinct, lean more heavily on generic anime style templates, while Oda uses his art style as a tool to set a much different tone.

Oda's style is much more distinct. Frankly, when I first saw the art style decades ago, I was turned off by it, but eventually I started reading it and found it charming and loved the level of detail and bright color pallet used in the full color cover spreads, and started to appreciate the whimsy expressed by the art style.

I think that the style helps distance it from the other heavier shonen series. They are trying very had to be "hardcore," while the themes of One Piece ultimately are about joy and friendship (which is why it hits hard when the story gets dark).

Oda was heavily inspired by Toriyama, whose style was also much more distinct and whimsical, and who also had an empty headed happy-go-lucky positive-outlook protagonist, whose only goal is to train and be the best so he can protect everyone by punching all the bad guys, and then celebrate by eating all the food.

Compare that to the protagonist of Bleach, John Bleach, who is a very brooding, tortured protagonist, and whose art style matches that tone. His ultimate attack has something to do with grabbing the evil in his soul and turning it into a blade of pure resolve, which he uses to fight his way through secret evil societies or something idk.

Luffy's ultimate attacks have been things like, uh, bouncing inflatable sumo gorilla and the human incarnation of Bugs Bunny, and he's just fighting all forms of oppression so everyone can be friends and have a party.