Because such artwork it's on a ownership limbo of sorts, Nintendo owns the character, nintendo owns the original sprites, but Nintendo did not create the fan sprite, so despite their ownership of the IP, they cannot claim ownership of the sprite, Nintendo could potentially sue the fanartist over the creation of the sprite (but maybe would require the fanartist or DA to profit from it), but Nintendo does NOT have the rights of usage over the sprite.
To use the sprite in a game would require a legal arragement or compensation with the creator of the sprite, even in a case like this where the accidental use came from the incompetence of someone who couldn't be arsed to get the original asset.