This might be peripheral but I would imagine Tosa artisans would have been looked down upon simply for historical reasons dating back to the battle of Sekigahara in 1600 when the Chosokabe (ruled Tosa Han) sided against the Tokugawa and lost. Animosity between the samurai of Tosa and the Bakufu continued until the Meiji Restoration and collapse of the Edo Period in 1868 due to the treatment of the tozama (the great daimyo who had opposed the Tokugawa and their respective domains, Tosa, Choshu and Satsuma). These domains also happened to be the ones that fought to restore imperial power (not a coincidence) and many of the earliest, most prominent Japanese politicians of the Meiji Period were from these domains such as Japan's first prime minister Ito Hirobumi who was from Choshu and Itagaki Taisuke, who served with Hirobumi as Home Minister in Hirobumi's Second Administration was from Tosa.
It would therefore not surprise me that works from artisans in Shikoku would have been looked down upon in general, but particularly so if the work came from Tosa. Simply put, looking down upon Tosa with distrust was sort of the unofficial position of the Bakufu itself throughout the Edo Period. The Chosokabe in Tosa were also the most powerful samurai clan in Shikoku so to see this mistrust and animosity extended to Shikoku as a whole would hardly be surprising.