Well, there are a few reasons:
1. Animes/mangas in general aren't that popular in the west as they are in Japan. Perhaps it got more attention recently, specially with the One Piece live action, but it's a MUCH stronger market in Japan;
2. The more successful ones had better adaptations and stronger starts, I think. One Piece has a slow start and only "hooks" most people in Arlong Park (some may even start to like it much after that, in Arabasta or even Water 7, for example). In the other hand, Naruto has a much stronger start; while I think it's massively inferior to One Piece after the Sasuke rescue arc, the first arcs (Land of Waves, Chūnin Exams, Konoha Crush) are among the best.
I think that Dragon Ball Z came first to the west, and bloody action packed cartoons were everything a 90's/2000's kid wanted, although I personally prefer Dragon Ball. Pokemon was another thing, it was already successful due to the games for Game Boy;
3. The One Piece dubs were horrible back then. At least in my country, the dub was based in the 4Kids dub, so, in my view, it didn't seem worth watching at the time. Scanlation was still crawling in the early 2000's, in my country they were more focused in the "main hits" of the moment (Naruto, Bleach, Death Note, etc.).