The Samurai on horse-back had three main weapons, the first and foremost was the bow and arrow kyudo followed by the spear so-jutsu and as a last resort the sword was drawn kenjutsu.

All that changed when Samurai became urban dwellers and, moving around on foot and in close proximity to each other,  started to rely more and more on their sword skills then on the bow or spear.

When the Shogugawa Shogunate introduced standing armies of common foot-soldiers ashigaru the yari, because of its longer reach and ease of handling and, as it could also be cheaply mass-produced, became the weapon of the day.