With the reveal of Pikmin 3 Deluxe, Nintendo has taken the initiative to remove the original Pikmin 3 from the Wii U eShop. This is likely due to the Wii U version's reduced $19.99 price tag, while the upcoming Switch release will cost a full $60. Something similar happened with Donkey Kong Country: Tropical Freeze, as that game was also removed from the Wii U eShop following the announcement of a Switch port.
Digital codes are still being offered from select retailers, but Nintendo will not continue selling the original Pikmin 3 directly from their store. The webpage is still up, but purchases are no longer available.