I think that the main factors is the length of One Piece and bad anime adaptation (VERY stretched fights, and bad animation compared to HxH and Bleach). Plus, One Piece is about emotions and atmosphere and not really about pathos (pre timeskip), younger generation prefer "edgy" Bleach or "cool ninja" Naruto than emotional or warm moments. Thats why OP lost in the West to DB, Bleach, HxH, Naruto.