Naruto is more popular than One Piece WorldWide, by a lot.
I won't even talk about the popularity of Dragon Ball Z WorldWide.
And OP sells so much above DBZ is because DBZ was over a long time ago, everyone saw it in the past man, and Anime wasn't such a "Huge" deal as right now back in DBZ Prime days.
Naruto has the most interesting story to me out of all Mangas and Animes I've seen ( There's a lot ), so it is legendary in a way, I like Badass Characters like Madara, and the whole story is/was very well written, because currently, with Kaguya introduced Naruto is nowhere near Legendary in Story, it went DOWNHILL so much compared to it's prime. So Kishimoto has to step it up and End the Series in the right way.
On the other hand why is One Piece "Legendary". It has terrible, terrible drawing style, like everything is so ugly there, the story is kinda appealing, but get's repetetive, maybe that's just because the Anime/Manga is so old and it's been going for so many years and unlike Naruto it got real Boring real Fast. Also the Character telling there is strange, for example in my Eyes Luffy is nowhere decent Main Character.