Because he's a water that doesn't fight, he literally darks.
Because dark is about dirty fighting and he's a ninja. The whole point of the starters is a double type triangle. Grass > Water > Fire > Grass and Fighting > Dark > Psychic > Fighting. Also the RPG motif of warrior/thief/mage.
>Greninja will never be a badass Water/Fighting
>Chesnaught will never be condemned to permanent shitmon hell by being Grass/Dark
Fuck you Gamefreak for wasting perfect opportunities like this.
I think you posted the wrong pic, OP