One pair of katanas in the game?

[Gilraen]

The names of the Samurai and Ninja weapons in XI are actually a mistranslation. In the original Japanese the Samurai use Katana and the Ninja use Kogatana (or small katana). For some strange reason, the translators decided to follow the sword naming conventions, with Great Katana and Katana like was used for Great Sword and Sword. It's a small difference that's had lasting consequences, apparently.

As for why the only Japanese daggers in the game are the Ninja relic, well that should be immediately obvious. Ninja isn't a Job native to Eorzea so why would there be random Japanese daggers sold by vendors?