The simple answer is that It's just leagues better structured than the other two manga you mentioned.
I'd argue that overall Naruto and BLEACH had stronger beginnings, One Piece only gets "good" around after chapter 80ish (Arlong arc), but One Piece is just more consistent. And weirdly enough One Piece was super popular in the TOCs since its very first chapter (it basically was #1 since the very first chapter, I'm not exaggerating you can look it up). I do find that kind of weird as I feel the first arcs of One Piece are kinda slow and just ok.
Also you can notice a very sharp decline in both Naruto and BLEACH... And One Piece never really got a super sharp decline.. I mean sure it has some not-so good arcs (And a couple of just weird arcs.. Like the Davy Back Fight arc which was totally unnecessary, pretty much filler lol).
Also as
@OniluapL said it's probably because it just has less flaws overall than other shonen, it's not perfect of course, but it does what it wants to do better than most, you really get a sense of "adventure" and "exploration" when reading/watching One Piece. I don't really think One Piece fights are the best ever, I enjoy the world building, mysteries and gags more than 95% of the battles.
I think a lot of people do critique One Piece, 15 or so years ago Naruto/BLEACH fans always complained about how childish it looked.. (I dont really browse many forums nowadays, but I imagine that's, along with its length, is it main critique nowadayas too). And sure I can agree it does look goofy, childish, even weird sometimes. There are other critiques about the story like no romance, no deaths and stuff like that.
All of this is my opinion, except One Piece dominating the TOC since its very first chapter, that's fact.
To finish this I'd like to point out another thing One Piece has done that both Naruto and BLEACH (And virtually every other shonen I have read) failed to do is that the different "groups" within the world of One Piece feel more genuine.
Spoilers for Alabasta in One Piece:
So Alabasta is the first mayor arc of OP, it lasts from around volume 18 to 23ish, and you have 5factions basically: Baroque Works, The World Government (Marines), The StrawHat Pirates, The rebel army and the Alabasta Kingdom army... Just with that alone its more complicated than the pretty two dimensional groups in most other shonen (BLEACH for example sure Soul Society was great.. but every other arc the bad guys were bad because.. They were bad basically).
In Alabasta you have Kohza who was the princess of Alabasta's best friend back when they were kids, the whole Baroque Works conspiracy which saw a pirate that was supposed to be allied to the world government taking over an entire nation, Vivi the princess of Alabasta infiltrating Baroque Works to gain intel, Nico Robin who betrays Crocodile the leader of Baroque Works, and... sure the strawhats are and will always be the good guys, but every other faction is more complex than any other shonen I have read. Every character had its own motivation! And the most interesting thing is that for example we learn more about Robin's motivations a couple of arcs later.
Some other examples of how intricate the factions in One Piece are
(Minor Spoilers for the rest of the manga):
Kuma of the revolutionaries, which oppose the world government, was called Kuma the tyrant, why was that? Was he, who opposes the bad guys bad himself in the past?
Arlong is a Fishman who did some horrible stuff, he thinks Fishman are superior to humans.. You feel he is a racist PoS, only to learn 500 or so chapters later just how cruel humans are to Fishman actually slaving them and stuff. (Keep in mind you don't actually learn Arlong's motivations in the arc he appears, only 500 or so chapters later, retroactively making the Arlong arc better and building its character "So I kinda understand why he was like that... He's still a PoS")
Meanwhile Naruto had... a war between puppet zombies and humans lol And BLEACH had a war between characters that really only went to war because they had to..
And I don't want to start a OP vs Naruto vs BLEACH war or anything, but even fans of those manga accepted the downward spiral of an ending both series had. it's like a consensus even among fans.