Because swordsmanship in One Piece is more developed and expansive than most other Shounen.
In Naruto, swordsmen are fodders and ninja are peak beings while it's vice versa with One Piece where ninja aren't that strong while most top tiers are swordsmen.
Swordsmanship is one of the strongest fighting styles in One Piece because of how lethal and it gives you a wider range.
And it's traditional for pirates to use swords, guns and the likes.
But Oda focuses on swords more due to Japanese influence.