Because it's not mythical or legendary... was it mythical and it's just not as of Gen 6, or was there a special event Zorua/Zoruark, what was the reason? Or maybe they just thought that Pokémon was really cool. Please help, and thanks in advance?
It was probably just to build hype for Gen 5. They may have seen Zoroark as the next Lucario (who was the star of the last movie), and as Lucario became a fan-favourite, they wanted to do the same with the edgy fox. It definitely worked to some extent -- Gen 5 sold pretty well at first and Zoroark was one of the more popular designs.
Hope I helped!