Intriguingly, it seems that Nintendo is going so far as the discontinue a series of bundles, some relatively recent, including all black models. Once these systems are sold out in Japan, they'll be gone forever.
Listed as discontinued on Nintendo Japan's hardware page are the original launch models, family bundles that include Wii Party U and New Super Mario Bros. U, along with Dragon Quest X and Monster Hunter 3G: HD Version bundles. The latter two being discontinued is arguably unsurprising due to them being third-party specials, though the removal of the family options is interesting. The options being actively promoted on the hardware page are Wii Sports Club offers or the core standalone system.
From a manufacturing and logistical perspective, and no doubt taking into account consumer preferences and buying habits in different regions, it makes sense for Nintendo to prioritise white models in Japan and the black equivalents in the West. Reader HCK, who got in touch to tip us off on this, states that the black models are very hard almost impossible to find at retailers in Japan. A look at Amazon Japan does show standard white models easily available, while black systems are typically out of stock or in very low numbers from third-party distributors.
Discontinue the White 32 GB version of this thread if old.