>Why did it failed as a shillmon?
I see this question all of the time and it's such a weird question, because it implies that it's unpopular despite the pushed marketing. Regardless of how it was handled, Zoroark was a geniunely well-recieved pokemon and even place 2nd in the Google Polls. That fact that alot of people were disappointed in how it was obtainable in BW shows that there was a strong demand for this line. While it's popularity isn't on the same level as Lucario, that doesn't mean it needs to compete with Lucario to be successful.
A better question would be "Why did GF failed Zoroark as a shillmon?" It's clearly obvious that with it being apart of the Alola family in the anime, and it being given a new regional variant(also well-recieved) that has a anime short and being showcased facing Lucario in the new SV games that they're giving Zoroark more attention than it had before.