I'm vaguely aware of why shields were uncommon in Japanese warfare; due to the prevalence of archery, horsemanship and polearm fighting in post-Nara, pre-gunpowder Japan. That being said, Japan has consistently had a culture of one-on-one dueling, both as part of warfare and without. Furthermore, there are many contexts in Japan in which sword fighting outside of armor might be expected.

In Europe, the combined prevalence of heavy armor, polearms and missile-based warfare meant that shields became increasingly irrelevant on the battlefield, but a specific type of shield remained prominent up until the pistol became a common sidearm; the buckler. A small, straight sword such as an espada ropera, arming sword or rapier was commonly paired with a buckler in either formalized duels or for self defense. A buckler was useless if you were wearing full armor, but when you weren't, and you were only facing one or a small amount of enemies like a mugging or street fight, it was perfectly suited for deflecting blows and tangling or grappling. From what I understand, conditions that led to the proliferation of the buckler were also the same in the Edo period of Japan, and yet the buckler either never took on there or no similar defensive implements were used.

Was this simply because shields having been rendered obsolete long ago in Japanese warfare, no one iterated on the large shield to make it into the buckler, suited for dueling and personal defense? Or were there other factors at play that made it so that the buckler was rare?