I have to say, I'm a bit surprised the yumi replaced the warbow. Yumi was quite popular on horseback, and many people believe the unusual curve of the bow was designed for ease when firing from horse. The warbow, however, cannot be used on horseback.
And before someone jumps in as says "but Yumi's were used before the introduction of horse-archery in Japan!" I'm aware. The fact remains though, they were commonly used from horseback once horse-archery became wide-spread.
If anything, why wasn't the strong-bow replaced with the yumi? I haven't ever heard of a historical bow referred to as a "strongbow." There are bows referred to as a "war bow."