This boils down to giving the people what they want. When we think of fantasy, we think of swords and dragon slaying. We think of strong knights that ride with their swords ready to do some damage. We think of armies of sword wielding men.
In short, the sword is the overwhelming most popular weapon. There are a lot of different ways to make a sword so designing new weapons can be easy. There's not as many ways to change up the look of an axe, spear, or staff.
The sword is accepted as a powerful weapon in both japanese and western culture. The only other one is the gun, and it doesn't work as well with the fantasy aspect. Swords and hot chicks sell, and that's what ff gives you every time.
_________________
Poem: A Jimi original
Image: Mia's great work!